Carburetor Tuning the Scientific Way
How to Tune Your Holley Carburetor
and Other Fuelish Ramblings
By ASE Certified Master Auto Mechanic, Engineer, and Inventor, Gerard Forgnone

Buy a book on Holleys or other carbs here:

I'm purposely making this one long page, so you can save it to print or view later. Be sure to save the illustrations separately. The opinions expressed here are my experiences, and your experiences may vary. I don't put myself forward as the world's leading expert on Holleys, but I do have some helpful ideas. In some cases, I may not be able to answer specific questions, but I'll do my best. I am moderately experienced in 4160/4150 Holley carbs, and can answer most questions about them.

I know very little about other brands of carburetors and have no information about them, only general operating principles.

I have NO experience with any type of supercharger or turbocharger, so please don't ask me anything about your blower or turbo applications.

The following few paragraphs is a set of rules I had to create because people were emailing me without enough information or asking really stupid questions that I had no way to answer.  The rules didn't just happen overnight, nor were they something I wanted to do.  But I kept getting questions from people who didn't do any reading or they asked a question I didn't have any way to answer.  Look at the rules, and think about the stupid question that I got asked 50 times, and you'll understand why I had to write them.

Before emailing me with your questions, you must read and abide by these rules:


Many of your questions are already asked and answered on this page. I keep getting loads of emails from people who want me to answer questions that are already answered here, and they are even claiming to have read the page.  It's obvious to me that they haven't, and this behavior is beginning to really take a toll on me.  I'm getting REALLY ticked off at questions that you can answer yourself it you'd just take the time to read the page!  If I feel that you're asking a question that's already been answered here, you will get a form letter response from me telling you to read this page. I'm not kidding!

No. 2: BE SURE TO REMOVE YOUR SPAM STOPPER! Or provide your correct email address in the body of the message! Some people have asked questions, and I have spent several minutes composing an answer. Then I send it off and it bounces because the email address is incorrect. If you want help, provide your correct email address!

No. 3: Send the part number of the Holley carb! This is the number that follows the letters "LIST" on the airhorn of the carb. I must know what you are working on in order to help you!

No. 4: Send some details about the size of engine, modifications to the engine, size of car, etc. A dragster with a 454 will require vastly different carburetion than a motorhome with a 454! You don't have to get really specific, but some basics such as the weight of the vehicle, engine mods, trans choice, rearend gearing, and intented use are necessary.

No. 5: When sending replies back to me after I've replied to you, retain all previous correspondence in the email, so I can reread it. I get 20 to 30 emails EACH DAY, and I just can't remember the particulars of your situation.

No. 6: Use only Holley parts! If you use a rebuild kit from someone other than Holley, I will not help you! Many problems are CAUSED by poor quality replacement parts. If you are having problems after a rebuild with non-Holley parts, rebuild it again with a Holley kit, following the instructions in the kit.  I'm not kidding!  Read this from a guy who wrote me in June- July 2000:

"Thanks for the info - you have been helpful. The first
 kit I bought - Sierra brand - cost $64 - and didn't solve
 the problem. I priced a Holley kit for $40. Don't know
 what the difference is yet or why it is $24 cheaper..."

"This is just a follow-up thank you note for you. You can read the
history of our correspondence below but to make a long story short -
I rebuilt the carb using the Holley parts - and it works beautifully.
Thanks for your help.
Randy R."

No. 7: Do not ask me about specific jetting, power valve, and accelerator pump recommendations for your vehicle! There is no way on this Earth that I can give anyone specific recommendations for jetting, power valves, or accelerator pump settings, because only YOU can drive the vehicle! YOU must determine whether your application needs to be richer or leaner than stock. You should begin from the stock jetting, because it's a good baseline. This page is designed to help you figure out whether to lean or richen certain circuits. If you ask this question, you will be sent a form letter telling you to re-read this page again.

No. 8: Do not use all caps! Please try to use some punctuation, and break up your questions so I can answer them easily. DON'T YOU AGREE THAT THIS IS HARD TO READ WHEN SOMEONE JUST GOES ON AND ON LEAVING THE CAPS BUTTON ON AND NEVER STOPPING THEIR SENTENCE IT JUST RUNS ON AND ON AND IT'S HARD TO READ DON'T YOU AGREE I THINK YOU WOULD AGREE... Enough said.

No. 9: If you pay someone to do work on your carb, or have purchased a brand new carb, ask them your questions.  I do this for free, and I don't appreciate people who don't seem to mind paying someone for work, but then "cheap out" and expect me to work for free.  I like to help those who help themselves, not people with deep pockets who think they can get something for free.  I also don't like to second guess someone else's work.  They may have done something unusual to your carb, and it wastes my time trying to second guess their mods.  This limitation also applies to those of you who buy a brand new carb, which comes with a WARRANTY, and then expect me to do your work for you.  A new carb will come with instructions, and a phone number to call if you have problems.  USE THEM!

No. 10:  Do some diagnosis BEFORE asking me your question. I'm getting really tired of drawing information out of people whose cars aren't running right or aren't running at all.  I keep getting the feeling that everyone wants me to reach through the computer and fix their cars, without their having to do anything.  At the MINIMUM, let me know if your spark plugs are clean and dry, or black and sooty.  If they are clean and dry, you may have a vacuum leak.  If they are black and sooty, you have too much fuel, caused by incorrect fuel level (too much fuel pressure, bad needle/seat, bad float) or a blown power valve (try a new one).  If you haven't done this amount of diagnosis prior to writing, you'll get a short email back from me referring you to this page again.  If your car doesn't run at all, DON'T EMAIL ME BECAUSE I CAN'T HELP YOU!

No. 11:  This is a free service. If you folks out there keep sending me emails that I can't decipher because you're not following these rules, I'm going to have to quit answering your questions, because of the time involved with doing this. I really enjoy helping people out, but I refuse to extract required information from you.

Having trouble tuning your 4 barrel Holley? This guide can help! If you know enough to be dangerous when tuning your carb, but not enough to get it right, you need to read this! You will need to know the basics about how to work on your Holley, and a book or two will really help a bunch, because of the pictures and part numbers in them. This guide goes farther than the commercially available books in that it helps you determine when your carb is rich and when it is lean.

No more guessing!

The special hardware you need to diagnose your carb's performance can be purchased for under $50! This amount may be all you have to spend, depending on what tools and instruments you have in your garage right now. Some of the instrumentation can be borrowed from a friend. This guide outlines exactly what you'll need and lists part numbers for those pieces that are not normal in a well-equipped garage.

Author's Background

I started working on cars before I could legally drive, and rebuilt my first V-8 (a Chevy 283) at age 14, at Pop's Chevron service station. Pop started me off rebuilding Ford Autolite carburetors, one of the simplest fuel mixers ever made. I was the Valedictorian of my high school, graduated from automotive school, and graduated from the most prestigious public engineering school in the nation, Cal Poly, San Luis Obispo, where I received a Bachelor's degree in Mechanical Engineering Technology. After college, I worked in aerospace, eventually becoming a real, live rocket scientist at Vandenberg Air Force Base. After tiring of that, I became an inventor and United States patentholder, and am currently making a living off my wits and skills as an inventor and entrepreneur.

Common Problems

Lots of folks have been emailing me with some common problems, so here are some tips for fixing these common problems.

Gas is flowing out of places it shouldn't.

This is most likely due to a bad float valve, bad float, or excessive fuel pressure.  The first step is to check the fuel pressure, which should be between 4 psi and 7 psi.  If the fuel pressure is correct (or you have no reason to believe that it has changed on an existing setup such as an old stock mechanical pump), either the float has sunk, or there is a problem with a float valve.  If you have black nitrophyl plastic floats on a carb over 5 years old, the float may be soaking up gas and sinking.  Also, if the carb is near, or over, 5 years old, the float valves (inlet needle and seat assemblies) may need replacing.  Float valves have viton rubber tips (most cases) and rubber o-rings that can go bad, especially when they sit around for long periods, as on an old car that hasn't been driven, or a boat stored for the winter.  I suggest you purchase a Holley rebuild kit and rebuild the entire carb, because the cost of two float valves is almost as much as the cost of the entire rebuild kit.

How to hook up electric chokes.

Many Holleys come with electric chokes.  I don't like them.  This is due to the fact that they cool down faster than necessary when you shut the engine down.  Hot air chokes are better, but if you want to use an electric choke, here is what you need to know.

Make sure there is airflow through the choke spring housing.  This is the most common error people make, and it ends up burning the bi-metal spring.  Holleys draw air through the choke spring housing to keep the spring from overheating.  You must make sure that there is a clear path for the carb to draw a little bit of vacuum through the housing (there is a small vacuum passage behind the housing), and that the air gets filtered somehow, either with an external filter, or through a hookup to the bottom of the air filter housing.  In the second photo below, there is a brass compression fitting at the left.  That is the air inlet to the choke coil housing.  This particular one has a little screen in it to keep out the big debris, but to make it work better, run a copper or aluminum 1/4 metal tube down to the exhaust manifold to draw in hot air and filter out the big chunks.

Here is a pic of the vacuum passage going into the carb.  There is a cork gasket on the choke housing that matches up to this port.

You will need a source of 12 volt power to heat up the bi-metal spring.  This source must be energized ONLY when the engine is running, so make sure you don't hook up your wire to the accessory circuit.  This, again, is how many bi-metal springs get burned up, because there is no airflow through the choke housing when the engine is not running.  The hot side of the 12v wire hooks up to one tab on the choke, and the other tab of the choke goes to ground, usually one of the choke housing mount screws.  I highly recommend putting a fuse of some sort in the line, because a failure of the bi-metal spring could cause an overload of the wire and a fire if there is no fuse in there somewhere, and typically ignition circuits are not fused.

How to switch from automatic chokes to manual chokes.

Automatic chokes and manual chokes do the same basic thing on Holleys, but use different linkages to do so.  See the two photos below.  If you swap one for the other, MAKE SURE you get the corresponding linkage for the end of the throttle shaft, because auto and manual chokes use different fast idle linkages.  Auto chokes have an open vacuum port on the body of the carb that needs to be plugged/blocked for use with manual chokes.  On manual choke carbs, Holley typically uses a flat adhesive sticker to block the port, and you can use a piece of duct tape or other strong, flat adhesive backed tape also.  The reason it needs to be flat is that the manual choke structure goes pretty close to the vacuum port, but not quite onto it in some cases.  If you're going from manual to auto choke, make sure you uncover this port and put the little red cork ring gasket in there to seal off the fitting to the choke housing.

Automatic Choke and Shaft Linkage

Manual Choke and Shaft Linkage

Choke vacuum cover after rebuild or when converting to manual choke.

How to check fuel pump output.

Your carburetor requires 4 to 7 psi of fuel supply at all times, under all conditions.  Many "carburetor" problems are actually fuel delivery problems.  Bogging and not enough power can be caused by too little fuel, and overflowing can be caused by too much fuel pressure.  Before blaming your carburetor for problems you MUST check the fuel delivery while driving under whatever condition you use the vehicle.  This means rigging up a fuel pressure gauge outside the passenger compartment and going for a drive.  If your fuel pump doesn't deliver 4-7 psi at all times to the supply line at the carburetor, then you need to examine/replace ALL the lines and hoses (this includes the hoses at the tank) and possibly replace the fuel pump if it isn't delivering enough fuel.

Vacuum ports and what they do.

Not all Holleys have the same ports, but you can tell by location and size what each one does.  Any port at the very bottom of the carb (in the aluminum baseplate) will pull manifold vacuum, which is strongest at high vacuum situations such as idle and slowing down, and weakest at low vacuum situations such as wide-open-throttle (WOT).  Common manifold vacuum connections at the bottom of the carb are PCV (usually a 3/8" diameter tube) and EGR/distributor retard/smog device hookups (usually smaller tubes under the primary fuel bowl).  The other type of vacuum connection on a Holley is ported vacuum, which is weak at idle, strong at cruise, and weak at WOT.  This one is always (if supplied at all-some competition carbs don't have this port) on the passenger (US) side of the carb, above the idle mixture screw, and is generally used for distributor vacuum advance.

How to switch from vacuum secondaries to mechanical, and from mechanical to vacuum.

You can't!

Vacuum secondary carburetors have vacuum passages built into the body of the carb to provide vacuum to the secondary diaphragm, which operates the secondaries when there is enough airflow going through the primaries.  Vacuum secondary carbs do NOT have separate secondary accelerator pumps, and never have had them.  There is no provision in a vacuum secondary-equipped carburetor for an accelerator pump passage.

Most mechanical secondary carbs have passages for secondary accelerator pumps, and none have vacuum passages for vacuum secondaries.  Therefore, it is impossible, due to the passages built into the body of the mechanical secondary carburetors, to switch a mechanical secondary carb to vacuum secondaries.

Please don't tell me how you can put a screw into the linkage of a vacuum secondary carb to make it mechanical.  This does NOT work properly, and will only cause headaches.  If you want a mechanical secondary carb, you must start out with a mechanical secondary carb.

Boats and other seasonal engines.

Boats (and other seasonal vehicles like motorhomes) have a unique problem in that most are stored in the off-season, and the fuel is left to dry out in the carb.  Rubber parts, including float valves, accelerator pump diaphragms and power valves, can dry out and stiffen.  When fuel is added at the start of the season, these components can fail, causing leaks, fuel level and richness problems, including loading up, plug fouling, and overflowing (which can be very dangerous on a boat).

If you experience any type of carburetion problem on a boat or other seasonal engine, I recommend a full cleaning and rebuild of your Holley carburetor and replacement of all rubber fuel lines immediately and then each season afterwards.  I have experienced the results of dried out fuel in the small idle passages of my own carb, so you will need to fully disassemble and clean all the passages on the carb, because simply replacing the parts without cleaning the carb will not make the result 100%.  While it may be expensive, a boat that won’t run on your first outing of the season can be even worse, considering the amount of prep time, transportation cost, and how much your family will be let down if the boat won’t run right.

Inconsistent idle speed.

Many people lately have been asking me about idle problems where they will set the idle at 800 or so, then when they drive the car or boat, the idle only comes back to 1500 or so.  When they blip the throttle, it will come back to 800.  This problem is very likely NOT the carburetor, but the linkage from the pedal to the carb, especially if the car (or boat) has a wire cable linkage.

To diagnose the problem, disconnect the linkage at the carburetor, and manually move the throttle with the engine running.  If it comes back to the idle set point, then the problem is in the linkage, which will require repair or replacement.  If it doesn’t come back to the idle set point, the problem is in the carb, possibly in the choke’s fast idle linkage.  If the linkage is dirty, spray it with carb cleaner.  If you're using a spacer or adapter, those can cause interference with the throttle plates, and will need to be fixed.

Occasionally, an engine will have a poor ground, and electricity will seek ground through the throttle cable, making it hot enough to melt the plastic housing.  If you see evidence of a hot throttle cable, be sure to check your engine to chassis ground.

Inconsistent idle speed can also be caused by a sticky mechanical advance in your distributor, a particular problem on boats, where corrosion is a major factor.  A visual inspection of the advance mechanism is highly recommended.

Another cause of inconsistent idle speed is sticky throttle valves, especially the secondaries on vacuum secondary carbs.  Stickiness can be caused by excessive wear, but is most likely corrosion or gumminess from disuse, as in boats and other seasonal engines.  The secondaries are held closed by two devices: The spring inside the vacuum actuator, and the link that goes from the primary throttle lever to the closer slot on the secondary throttle valve.  In some cases, the actuator spring is relatively light, and doesn't exert much force.  If the closer link is not bent just right to touch the slot enough to fully close the secondaries, you will experience a situation where the idle speed will not come down after the secondaries are used.  You will need to clean the throttle plate and/or rebend the closer link to get the secondaries closed properly.

Sooty spark plugs and fouling.

Having spark plugs foul out with dry sooty deposits is a direct result of excessive fuel.  This is usually caused by two things: Improper fuel level and/or blown power valves.  The fuel level problem could be a misadjusted float, excessive fuel pressure, bad float valve, or bad float.  If the fuel level is ok (just licking at the bottom of the sight hole), read more below on blown power valves.


Holley has really updated their website, and has now included installation instructions for you to download, print, and read.  These are very informative, with lots of photos.  If you're working on your carb, print one out, and you'll get a lot of answers.  The first page of listings has the sheet for the common 600 cfm LIST 1850, and the 750 vacuum secondary LIST 3310.  The second page has a bunch of double pumpers, starting with the LIST 4776.  Here is the Holley link:

Common Questions and Answers:

Question: Is the Holley spreadbore better than my Rochester Quadrajet?
Answer: Not at all.  The Holley spreadbore carb is a poor imitation of the Rochester Quadrajet.  Here's why.

The Rochester Quadrajet is a modern design, and uses some very forward-thinking features such as a central float bowl with one float, vacuum-operated metering rods for power enrichment, an air-door controlled secondary, and no gaskets below fuel level.  A couple of disadvantages are its sliding cup accelerator pump, and hard-to-find parts.

So, how does this relate to Holleys, since this is a Holley page?  The design of the Holley spreadbore replacement leaves out many of the good features Holleys are famous for, and leaves off most of the good features of Rochester Quadrajets.

The Holley spreadbore lacks the following features, which are standard on its squarebores: Externally adjustable floats, standard accelerator pump nozzles, standard gaskets.

All Holleys lack the following features, standard on a Rochester Quadrajet: Metering rods for power enrichment, air-door controlled secondary.

Since the Holley is lacking in so many things that the Quadrajet excels at, I recommend to anyone who is thinking of putting a Holley spreadbore on any engine: Don't!  If you have a spreadbore intake, use a Rochester Quadrajet.  If you really want a Holley, change the intake manifold to use a squarebore Holley.  Don't use adapters, either!  They are proven power robbers.

Basic Theory

Carburetors are really just dumb fuel and air mixers. Any source of airflow through a carb will draw fuel. This is why a carb for a 460 c.i. engine will also work on a 260 c.i. engine. The performance may not be optimum due to signal strength or restrictions, but if the jetting is close for one, it will be close for the other. 2 examples: I took the 600 cfm carb off my 390 truck and put it on my 4 cylinder Pinto. The jetting was right on! My Pop took the 390 cfm Holley off my Pinto and put it on his 390 powered 62 Thunderbird. The mixture was right on. This is why it is important to know what jets your carb came with, and to start out with those jets.

The factory determines jet sizes using several factors on each carburetor, including the air bleed size, the venturi size, booster venturi size and shape, and the power valve channel restrictions. Street carbs and performance carbs of the same cfm rating will be jetted differently, too, so you can't always use the jet rating for different models of the same cfm. Even upgraded models of the same carb will have different jet sizes. So if you find that your car will not run properly with a jet that is within 5 sizes one way or the other of the factory recommendation, you have problems somewhere else, like a bad or misadjusted float, bad needle and seat, a vacuum leak, a dirty carburetor, or a plain old worn-out carburetor.

My first recommendation is to BUY A BOOK OR TWO ON HOLLEY CARBS, or YOUR PARTICULAR BRAND! You are going to need at least one for the photos and part numbers. I own at least 9 different books on Holley carbs, several general repair manuals that cover the Holley, and have cut out many articles from Car Craft, Hot Rod, and other magazines.

We now have an association with, and have selected a few Holley books for you to choose from.  Click here to check out the books and videos:

BOB2000 book and video store

Here's an important word of advice:

Do NOT buy cheap rebuild kits for your Holley carb! All the cheap stuff is junk, especially the needle and seat assemblies, and you will not save any money in the long run! Some of these off-brand kits actually cost MORE than the Holley brand kits!

One thing you CAN do to save money when buying Holley rebuild kits is to buy a kit from a SIMILAR carb in the Holley line. Example: I ran a really obscure 0-8007 390 cfm 4 barrel on a 4 cylinder Pinto. The kits for this carb are expensive because Holley doesn't sell very many. BUT, most (if not all) of the parts in the 8007 kit are identical to those in a number 1850 Holley kit, the very common 600 cfm 4 barrel. That kit is the cheapest. The same goes for the 428 CJ carb on my Fairlane. The specific kit for it is exhorbitantly priced, but all the parts are the same as the venerable 3310. The things you really need to watch for are the type of accelerator pump passage used, and the baseplate to body gasket, which varies according to cfm rating. Always make sure you get the proper replacement by matching the new one to the old one. Sometimes, Holley will give you the WRONG BASEPLATE GASKET in a kit. This happened to me once on an 850 double pumper.

Carb Kits

Each Holley carburetor has a specific rebuild kit for its number.  If you are working on a relatively obscure carb, the specific kit can be quite expensive, upwards of $100, which may be more than the carb is worth.  Holley offers two generic "Fast Kits" for their squarebore four barrel carbs, one for carbs without a secondary metering block (most vacuum secondary carbs), and one for carbs with a secondary metering block, including ALL squarebore double pumpers.   Below are part numbers and links to a common retailer, to which I am not affilliated.  If those links don't work, or you have a different retailer, use the Holley part numbers listed.

Without Secondary Metering Block: Holley PN 37-1542  (less than $30)

With Secondary Metering Block: Holley PN 37-1544   (less than $40)

Carb Theory

There are several circuits in your carburetor. Each circuit flows at different times, and some circuits flow even when you think they don't.


Most floats on Holleys are externally adjustable, except the Quadrajet replacement carbs, series 4175, most late-model smog carbs, and many marine carbs. The standard Holley has a sight hole on the passenger side. In order to set the floats on a Holley with externally adjustable floats, remove the sight plug with the engine off. Start the engine. If the gas just pours out all over the place, shut the engine off right away before you start a fire. Your floats are obviously too high. The way to set standard Holley floats is to unlock the screw with a large flat tip screwdriver, then move the 5/8 hex nut clockwise (CW) to lower float level, or counter-clockwise (CCW) to raise the float level. You're actually moving the whole float valve assembly up and down in the float bowl.

Click here for Holley's webpage on how to adjust floats.

The proper level of fuel is for the fuel to just touch the bottom of the sight hole without running out. Clear sight plugs are available, but I can no longer recommend using them, as they have a tendency to break off right at the o-ring.

If the fuel does not just trickle out of the sight hole when you rock the car back and forth lightly, adjust the float up until it does. If you have a problem with gas just rushing out all over the place, or if you have a carb that drips fuel from the booster venturis at idle or when the engine is off, then you likely have a bad needle and seat assembly. These are removable from the top without having to remove the float bowl, a very fine feature of the Holley.

You'll hear this often from me: Do NOT use float valves from anyone but Holley, or maybe the Carb Shop, Barry Grant, or some other reputable racing supplier. The float valves that come in non-Holley rebuild kits available at auto parts stores are JUNK!!!!!! They often require you to discard the Holley nut and lockscrew, and they often have strange adustment procedures. DO NOT USE THESE!

Do not move the secondary float bowl to the primary and vice versa. The rear float bowl level is lower than the front to keep fuel from pouring out into the carb throat under heavy braking. If you have a car that accelerates well, Holley offers vent whistles and jet extensions that solve the problems that occur during heavy acceleration and braking.


The idle circuit supplies fuel when the throttle plates are open only slightly. Mixture screws let you adjust the mixture of this circuit. On a Holley, the fuel comes out a little hole below the throttle plates in the baseplate. There is an idle circuit in the rear barrels of your Holley, too. Most are not adjustable, but Holley put it there to keep the fuel flowing through the rear bowls so it doesn't get stale just sitting there. This is why you can't turn off the fuel flow to the secondaries. Doing so will mess up the metering system by causing a lean condition.

Idle circuit metering is accomplished by three things: Idle feed restrictions, idle air bleeds, and the idle mixture screws. Sometimes you can see the idle feed restrictors in the metering block. In some cases, they are installed in the bottom of the emulsion tubes, and require surgery to remove. This is not recommended, except for experts.


The transition circuit supplies fuel between the idle and the main metering system. This is a small slot above the closed throttle plate in the baseplate. The amount of fuel is determined by the idle feed restrictions.

You can put restrictors in the body of the carb to limit the flow of fuel through the transition circuit after the idle feed restrictions. I made a pair from two 10-32 Allen head setscrews with a properly sized hole drilled in the center. I then drilled and tapped the proper passage in the carb, and inserted the restrictors. Now, I have an adjustable transition circuit that I can lean out. Richening the transfer circuit will require enlarging the idle feed restrictors.

When the idle and transition circuits work:

Depending upon the size of your engine and the rpm's it is running at, you can have flow from the idle and transition circuits well into the higher rpms. This is due to the restriction of the venturis. If you have a too-small carb, even at wide open throttle, you will have manifold vacuum. Manifold vacuum is what pulls fuel from the lower parts of the carb, where the idle feed and transition slots are. In the lower parts of cruise, your idle and transition slots will be providing a small amount of fuel. Any time you see manifold vacuum, the idle feed and transition slots will be flowing fuel, because of the vacuum under the throttle plates.

Main Metering:

The booster venturi (that thing sticking out into the throat of the carb) supplies the major portion of the fuel at cruise and power levels. Metering is accomplished by the main jets and air bleeds, and enrichment comes from the power valve.

Accelerator Pump:

Supplies fuel under pressure to compensate for losses in fuel flow when the airflow signal to the booster venturis diminishes when you punch it from a standstill, or when the airflow goes away during changes in engine load.

In Holleys, there are two pump check valves:

The inlet check is above the pump diaphragm, and may consist of a steel check ball held in by a bail, or a rubber umbrella valve. The better of the two is the rubber umbrella valve, since it is normally closed and provides a quicker shot, because it doesn't need to seat itself like the ball does. Just be careful not to put these rubber valves into harsh cleaning chemicals, as some rebuild kits don't come with a new valve.

Here is a pic to show the two different bowls.  On the left is the ball and bail, on the right is the umbrella valve, uninstalled on that bowl to show the smooth sealing surface.  Clearance for the ball and bail is .011 to .013 inch.  Use care installing a new umbrella.  Lube the pilot and pull gently through to the other side until you feel a light snap as the knob comes through.

The outlet check on most Holleys is below the squirter. It consists of either a small ball held down by a steel cylinder, or a sharpened steel cylinder. Either is ok, but a small ball by itself is not enough weight to prevent siphoning by the airflow going through the carb throat. I've seen this on my own carbs, when I didn't realize there was a heavier weight to go on top of the small ball.

Spread bore carbs do not use an outlet check under the nozzle, but rather have one in the metering block. In order to prevent fuel from being siphoned from the passage between the metering block and outlet, they use a special anti-pullover nozzle, which prevents airflow from coming close to the outlet and sucking the fuel out.

Power System:

Activated by the power valve in a Holley, this circuit supplies extra fuel to richen up the main metering system. This is a vacuum signalled valve that simply opens and closes at a preset amount of manifold vacuum. The rating is stamped on the valve: 2.5 up to 10.5 inches of mercury manifold vacuum. The lower the number, the later the valve opens, the higher the number, the earlier the valve opens. The metering for this system is provided by the two little holes underneath the power valve, called "PVCR's" or Power Valve Channel Restrictions.

Replacing the primary power valve with a plug:

If I could, I'd put a electrical shock device in your mouse right now to deter you from even thinking about plugging a primary power valve on just about any application. Think of the power valve as a switch that richens the mixture for heavy loads and accleration. If you plug that power valve, you will need to increase the size of your main jets for proper full power mixture to the point that your cruise mixture will be so rich that your engine will actually foul spark plugs. Too much gas is just as bad as not enough gas. The engine will be sluggish at low rpms, and just won't run right. If you have problems with power valves blowing out, fix the problem, or install one of the available power valve protectors. Summit sells one for about $8. All 4010 and 4011 Holleys, and many newer 4150 and 4160 series carbs come from the factory with this power valve blowout protector.

It is, however, recommended to plug SECONDARY power valves, due to the inconsistencies encountered with WOT, low vacuum operation, which is when the secondaries are open.  Basically, the only time the secondaries are open is at Wide Open Throttle (WOT).  At WOT, vacuum is very low, and the power valve is open, so it's really a waste to even run one in the secondaries.  In the vast majority of cases, you will need to jet up on the secondaries when plugging a secondary power valve.

Power enrichment on other carbs:

The power system on Carter/Edelbrock 4-barrel carburetors and Rochester Quadrajets is accomplished using tapered metering rods that run inside the main jets. The rods are held down by high manifold vacuum, which makes the fuel opening in the main jet small. As manifold vacuum drops under power, a spring under the metering rod holder raises the rod out of the main jet. Since the rod is tapered, as it moves out of the main jet, the fuel opening gets bigger, resulting in a richer mixture. This system is very effective and easy to tune, since the rod tapers can be changed and different springs installed for varied rates of movement. And just for the record, I don't agree with Holley's new advertising about the drawbacks of metering rods. There really are none. I wish Holley would use them instead of power valves, because metering rods supply a gradual increase in richness, rather than a sudden one, like what happens when a power valve opens all at once. And, metering rods don't blow out like power valves do occasionally.

Double Pumpers and Gas Mileage (or lack thereof...)

Have you heard people complain about the gas mileage they get with performance-type double pumper carbs? There is a reason that the 0-4776 through 0-4781 double pumpers get bad gas mileage. It's the jets! Surprised? Holley sizes the jets and air bleeds on these carbs so that they run on the rich side at cruise speeds. They make more power this way, at the detriment of gas mileage. These are competition carbs, and they are supposed to work this way.  Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations.

The way to do this scientifically is to measure the diameters of the openings of stock jets and PVCR's using drill bit diameters. Then calculate the total area of all the openings, and add them up.

A = 3.1415 x dia x dia / 4

Decrease the main jets for proper cruise mixture, and enlarge the PVCR's until you get back to the original area of all the openings. This way, your cruise mixture will give you good gas mileage, and under power, the engine will have proper mixture.

Graphical interpretation of these systems:

These bar graphs (not to scale) illustrate the fuel flowing through your Holley at different stages. Knowing what circuit is flowing at what time is very important to your tuning ability.

Fuel flow graph

Vacuum Secondaries

Vacuum secondaries allow a person to run a carb that most people believe might be too big for a particular engine. By playing with various secondary springs, you can tailor the secondary opening point and rate of your carb, and can even make them to where they don't open all the way. This allows you to use a carb that might be too big for your application. The diaphragm housing with the removable top is highly recommended. Trying to align the screw holes while keeping the spring compressed is a quick way to pinch off and tear the diaphragm. One or two torn diaphragms is enough to pay for the quick change kit. I know this from experience.

On some Holley's, there is a steel check ball in the passage that actuates the diaphragm. Leave it there. If you take it out, the secondaries are likely to flop open very quickly and give you a bog.

If your vehicle is heavy, like a motorhome or 4x4, you should always use vacuum secondaries, because you need to be able to keep the secondaries closed during cruise.  Since these vehicles are so heavy and often use high rpm rearend gears, your carburetor opening is often so large that many double pumpers will open the secondaries during cruise, causing horrible gas mileage.  With vacuum secondaries, you can change the secondary spring to keep the secondaries closed for better gas mileage, yet still have them open during passing and hillclimbing.

A tip about vacuum secondary diaphragms

Holley makes about 4 different diaphragms. Even though one may look just like another, the link length can vary. This makes your secondary opening rate all messed up, because the spring is not acting on the diaphragm correctly. If your link is too long, the secondaries won't open all the way, no matter what spring you have installed. If your link is too short, the spring may not even touch the top of the diaphragm, which is not good.

Here is a photo of 3 different vacuum secondary diaphragms.  As you can see, they are all different lengths!  Make sure you use the correct one, or you'll have problems.

How to know if your vacuum secondaries are opening

You can tell if your secondaries are opening by keeping them shut with a spring or other means. Test drive the vehicle.  If performance is worse with the secondaries held shut, then they were opening when operating normally.  Alternatively, tie a bread bag twist-tie onto the vacuum secondary shaft near the top where the shaft would move in to the housing, and see if it has moved after driving the car.  Winging the engine in neutral does not work!  Vacuum secondaries operate off engine rpm and load, not just rpm, and there just isn't enough airflow in a no-load situation to open the vacuum secondaries.

Secondary Diaphragm Assembly

Assembly of the vacuum secondary diaphragm, spring, and solid cover is difficult, to say the least, the most common problem being tearing of the thin diaphragm by getting it caught in the screws.  I highly recommend the Holley PN 20-59 Quick Change Cover, if you have a non-stock application where you'll be changing the spring to tune the carb.

If you don't want to use the Quick Change Cover, here is a method to install the solid cover to minimize the danger of catching the rubber in the screws.

Get four 8-32 screws and cut the heads off.  Obtain two free running 8-32 nuts.  Make at least two of the studs about 1-1/2 inches long, enough to engage the cover before the spring starts pushing on the diaphragm.

Place the diaphram in place, making sure to align the vacuum port opening.

Attach the secondary spring to the cover and install it lightly while holding the diaphragm shaft in place.

Hold on to the diaphragm shaft and push the lid down until it seats to the lower part of the housing.  Hold it all together with your palm holding the link and your thumb on the lid, then run the two nuts down and tighten them with a small wrench to hold the diaphragm in place.

Remove the two studs not used with the nuts.  Install the regular screws.

Remove the two studs with the nuts, and install two more screws.  If using a manual choke, remove one screw at a time and install the choke cable holder.

Check the condition of the diaphragm by either of the following methods: 1) Use this method on on clean metal. Suck on the opening and stick your tongue on the hole.  The linkage should not move.  2) Push the linkage up into the housing, and put your finger over the hole.  The linkage should not move.

Now that you've got the basics down, here is how you tell what circuit needs to be richer or leaner. You put an instrument in the exhaust flow that tells you graphically whether the engine is lean or rich. You then put a vacuum gauge on the engine to tell you what circuit is working at the particular time that you are lean or rich.


The first instrument is known as an O2 (oxygen) sensor. These sensors are used in electronically fuel injected cars as a signal telling the computer whether to supply more or less fuel. O2 sensors create their own electricity in the presence of heat and oxygen. You simply put the sensor in a hot spot in your exhaust flow, then read the electrical output.

There are a couple of ways to put O2 sensors in your exhaust: The trick way, and the cheap way.

The Trick Way

Buy an air/fuel ratio monitor from Edelbrock or K&N. Summit (1-800-230-3030) sells the Edelbrock system for about $130. This comes with an O2 sensor, weld-on bung, and a display panel with yellow, green, and red LED lights representing lean, good, and rich mixtures.

Racer Wholesale (1-800-886-7223) sells several different air/fuel ratio monitors. One has two channels for dual exhausts, and another has rich/lean and injector duty cycle for you FI guys.

The Cheap Way

Buy a single wire O2 sensor at your local parts house, make your own bung, and read the voltage with a digital voltmeter. The sensor that I bought is a Standard brand, number SG-12. The threads on this sensor are the same as a small-block Chevy gasket-style spark plug, so the bung can be made from one of those spark plug anti-foul adapters. Other O2 sensors use the large diameter threads of 18mm big Ford spark plugs. Just cut and fishmouth the adapter so that the sensor sticks into the exhaust flow. You need to put the sensor as close as possible to the engine so it gets hot and stays hot. Just make sure you route the wire so it doesn't get burned by the hot exhaust pipe. Weld the bung to the pipe, then drill and file the hole to clear the sensor.

Since the purpose of this sensor is just a guide to help you tune your carb, not run a fuel injection computer, if you can't get the sensor really close to the engine, don't worry, because it will still work for your purpose.  All that will happen is that your reading may go away during periods of idling.  On the same subject, don't worry about using a heated sensor, as the expense and complications involved are not worth it for carburetor tuning.  Remember, your eyes are using this data, and if it stops for a while, no harm is done!

Sensor installed in Pinto exhaust.

O2 sensor in Pinto exhaust

Sensor and welded bung:

Outside view of sensor

Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. I have found my car to run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary.

Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.

O2 Sensor output

Vacuum gauges

You will need a gauge to read the manifold vacuum on your engine. The more accurate the gauge, the better your results will be. The manifold vacuum is measured at the base of the carb or on the intake manifold somewhere. Don't tap into just one runner, though, as sometimes this will give funny readings. You need to check the signal that sees all the cylinders. Most Holleys have a manifold vacuum port in the front on the passenger side under the primary float bowl.

Road Tuning

Start by taking your carb apart and writing down the sizes of the jets, the actuation point of the power valve, and the size of the accelerator pump squirter. Put it all back together, check for leaks, then drive it until the engine is warm. In order to get good readings, you will need to drive at a constant speed of 45-55 mph, accelerate lightly, and accelerate heavily.


The best way to set the idle mixture is to lean the carb out until the vacuum just starts to drop, then richen the mixture by about 1/4 turn. If you have a bit of a stumble in very light, low speed operation, sometimes it helps to richen it up by another 1/4 turn.

If you are starting your adjustments fresh, begin at about 2 turns out.  Depending upon if you have a regular carb or a smog carb, your adjustment can be lean when screwing in (normal) or rich going in (smog style).  The way to tell the difference is the smog carb will have a little sticker on the metering block telling you it goes opposite of normal, and the smog carb will have blunt screws rather than the pointed ones on a regular carb.  If you have a carb with a normal system where the mixture leans as you turn the screws in, then when you screw them in almost to a light seating, the engine should die.

On a regular, standard idle circuit Holley, turning the idle mixture screws all the way in should kill the engine.  If your idle mixture screws do not respond to adjustments, you may have several different problems.  The most common is using the wrong carburetor for the application.  Smog carbs with reverse mixture screws (these are the ones with the little decals, telling you to turn the screws in for richer mixtures) only have a small band of adjustment built in.  Smog carbs work well on stock engines only, and you can't expect them to work well on a modified engine, especially one with a lopey cam.

Another common cause of non-responsive idle mixture screws is having too much (more than .040 inch) of the idle transfer circuit exposed.  The idle circuit allows a very small amount of finely metered fuel into the engine.  By exposing the transfer circuit too much, the gross feed of fuel coming out of the transfer circuit overpowers the fine amount coming out of the idle ports.  This is akin to adding a squirt gun's flow into a garden hose's flow...the fine amount of the squirt gun is so insignificant compared to the garden hose, that there is no way that adjusting the flow of the squirt gun will make any difference.  There are several ways to fix this problem:

1. If your engine is worn, or has a big vacuum leak, you should fix those problems before trying to crutch your carb.  Sometimes, with worn engines or one with a vacuum leak, the carb must be opened quite a bit just to get the engine to run.

2. Buy the right sized carb!  If you're trying to use a 600 cfm carb on a lopey-cammed 460, then you must open the primaries too far into the transfer slots just to get enough air into the engine to get it to idle.  A larger carb on this example would have a larger throttle plate, which would need to opened less to allow the engine to idle.

3. Open the secondaries a bit, and close the primaries a like amount.  This will allow more air in, without exposing quite so much of either primary or secondary transfer slots.

4. If opening the secondaries doesn't work, then you may have to drill small holes (1 per plate) in the primary throttle plates to allow air in while the plates cover the transfer slots.  This is a trial and error procedure, so start small, about 1/16 inch.  You should not have to go much larger than 3/16 inch.  Drill on the side opposite of the transfer slots, to help keep this added airflow away from the idle ports and transfer slots.

5. Check for a blown power valve.  See the section on power valves below.

If you notice a lean surge or throttle tip-in stumble due to an excessively lean idle, and you can't get the idle richened up, you may have a clog in the primary OR secondary idle systems, or both.  Most people don't know that there is an idle system on the secondary side of the four barrel Holley to prevent the fuel from getting stale.  If the secondary idle system is clogged up, no amount of cleaning on the primaries will get the thing to idle correctly.  You must clean BOTH the primary and secondary idle systems.  And since those circuits are very small, small amounts of debris or even varnish from disuse will clog the tiny passages.

Main Jetting

Going down a flat road at a rate of about 45 mph or higher will give you a good indication of your main jet sizing. Shoot for between 400 mv to about 700 mv. Since carburetors are not as exact as computer controlled electronic fuel injection, keeping at the perfect 400 mv will be tough. You always want to go a bit rich, as excessively lean mixtures will cause damage to your engine, create pollution, and give you bad gas mileage and performance.

You can do this without a meter, but it's a bit tougher. Start with the factory jetting and proper float levels. Drive it around for a while, noting if the engine surges at highway speeds. Take a look at the plugs. If they are sooty, you may need to lean out the main jets. If the plugs are white, you may need to richen the main jetting. Start stepping the jets up or down, one step at a time, and drive the car around for a day or two. Do not make large changes on your jetting. You can't just throw any jets in there because you think it needs to go one way or the other. It takes time to get it right, and you must change jets one number at a time.

Secondary Jetting

This is a much more difficult task to tame, due to the acceleration most vehicles provide when the secondaries are open.  When the car is acclerating hard, it is difficult to read any gauges, therefore, I can only recommend the trail and error, or dragstrip method to jet secondaries.  Go by the seat of your pants or take the car to a dragstrip to work on the secondary jetting.  If you, or a spotter, can see puffs of black smoke when the secondaries open, then it's too rich.  That's about all the advice I can offer.

If your carburetor has the small metering plate inside the secondary bowl, you can change to a secondary metering block, with screw-in jets, using a secondary metering block conversion kit, which does not come with provision for a secondary power valve.  Secondary power valves are really not necessary anyhow, and just add complexity and failure points.

For those of you sticking with the secondary metering plate inside the bowl, here is the assembly order of the gaskets, which can get somewhat confusing on the schematics.  First down is the thick gasket covering the whole body surface.  Next is the thin sheet steel plate.  Then the thin paper gasket.  Last is the metering plate with the calibrated passages.

Power Valve

Once you have the main jets set, it is time to play with power valves. Going up a slight incline at highway speeds, or accelerating slowly at highway speeds, you will notice the vacuum reading falling. As it falls, it will come to the opening point of your power valve. You can tell when the power valve opens because the meter will go lean for a while, then the valve opens, and the meter begins to show rich. You'll probably notice the power increase right when the power valve opens. I can tell exactly which power valve I have in my car by the vacuum reading when the lean misfire goes away under light loads. About 700 mv to 900 mv is a good reading for light loads with the power valve open.

If your PVCR's are too small, the meter won't go up high enough, and power will suffer because the engine is still too lean. You can drill out the PVCR's with a small drill bit in a fingertip drill bit holder. If the PVCR's are too big, you will see a big jump in the meter readings, and maybe a puff of black smoke. My Pinto's PVCR's were way too big, so I found a kit to let you install little Mikuni pilot jets into the metering block of your Holley to make the PVCR's smaller. The company went out of business after the owner passed away, so if anyone knows of someone still selling this kit, let me know!

Now that you have the power valve mixture correct, try some different power valves. I like a 10.5 in my Pinto, because it pulls about 14 inches of Mercury going down the road, and the 10.5 comes in pretty quickly when going up slight inclines.

Tip: Use a power valve that is about 2 inches of mercury below the LOWEST manifold vacuum reading you get on cruise and idle (in gear for automatics). If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis. If the power valve opens a lot while you're driving down the road, your gas mileage will suffer.

Blown Power Valve

You may notice a bad power valve because you will not see the jump from lean to rich when your vacuum gauge gets to the power valve setting. You will see a rich mixture at cruise, when the power valve should be closed. You may also notice a badly BLOWN power valve as an overly rich condition at idle. This occurs only if the diaphragm has a big hole in it that lets fuel from the float bowl into the power valve vacuum passage and into the engine.

There are some hand held vacuum pumps and adapters out there to check power valves. I just use my mouth after making sure all the gas is dried off. If you suck a lot of air through the diaphragm, or it won't hold a vacuum, then replace it.

Accelerator Pump

You'll probably need the help of a spotter on this, because things happen fast when you punch the throttle from a dead stop. If your meter goes rich right away when you punch the throttle, then it leans out gradually, you need a smaller pump nozzle to make the fuel come out slower. If the meter doesn't go rich right away, but gets richer later, you may need more fuel earlier. A bigger nozzle is in order. Try some different cams, too, but be aware that bigger is not always better. If your car still has a lean stumble with the biggest cams and the biggest nozzles, you may have to go with the 50cc "Reo" pump diaphragm and the corresponding cam. You must use the big black cam on 50cc pumps to get the full shot of the 50cc pump! If you put that big old pump under there but don't move the lever any farther than before, what's the use?

Also, don't put the 50cc cam on small pump diaphragms! You'll break them! Most manifolds will require a 1/4 inch spacer under the carb to run the big pumps. So if you add a spacer, don't forget to check your hood clearance BEFORE you slam the hood!

Another thing to think about when you start getting into the big pump cams and nozzles is to use the hollow nozzle hold down screw. This is due to the fact that the little passage cut in the side of the internal threads may not be big enough when you're way up in shot size, over .040, as recommended by Holley.

Accelerator Pump arm adjustment: The accelerator pump arm must not be loose at idle. The .015 clearance is measured at wide open throttle (engine off!). This is to prevent damage to the pump arm and diaphragm. You may need to adjust the external spring one way or the other to get it tight at idle, yet still have the .015 clearance at WOT.

Interchanging spreadbore float bowls with squarebore

I mentioned to one guy my distaste for spreadbore Holleys, and how most have non-adjustable floats.  The guy said, "Just change them."  Well, you can't, at least not with squarebore float bowls, because the accelerator pump passages don't line up.  This is just plain stupid on Holley's part, but it is the way it is.

This is a photo of a blue squarebore bowl gasket lying on a brown spreadbore gasket.  As you can see the alignment pin holes don't line up, but more importantly, the accelerator pump holes are off.

Here is that same brown spreadbore gasket on a squarebore bowl.  As you can see, the passage doesn't line up, and the gasket will let the accelerator pump shot go right back into the fuel bowl.

 Some more typical problem areas

Holley changed to a plastic accelerator pump arm many years ago.  A recent teardown has exposed a wear area where the arm contacts the pump cam.  This can cause an uneven actuation problem if it gets severe enough.  If you look closely at the pivot, you'll see that the groove for each style is different, so if you remove a plastic arm to use a metal arm, you'll need the corresponding pivot pin.

During this rebuild, I moved the float up and down to check for free movement, and was surprised that it wasn't free at all, but was hanging inside.  When the float was removed, a bent pivot was discovered.  These are very easy to bend, but won't bend on their own, so somehow, someone was careless in the past.


During teardown, I found that someone had drilled out the nozzle on the left.  Yes, these are both labeled as 031 nozzles!  This gets back to the modification of parts.  Once drilled out, there is no way the next person can know what they have, and sometimes, drilling can really mess things up, like it did with this nozzle.  I really don't even think Holley makes a nozzle as big as the one on the left.  The pump shot duration using this nozzle must have been terrible, with all the fuel coming out too soon.  Nozzle size only affects the duration of the pump fast it comes out, like a bigger hose with the same pressure.  The volume of shot is affected by the pump cam.

Fuel Filter Information

There are a lot of fuel filters to choose from, but one popular one is dangerous to use.  This unit is tightened together, and relies on o-rings to seal against fuel pressure.  As the parts age, the o-rings dry out and crack, causing leaks.  Additionally, the clear part is glass, and is not safe to use.

I recommend the one-piece clear fuel filter shown below.  These are inexpensive (less than $5) and have been in use successfully for years.  Always install the filter as close to the carburetor as possible, as this is the best location to avoid heat.

Well, I sure hope this helps you out! The best advice I can give is to get books and READ THEM! The guys who publish the carb tuning books are experts who get paid to do this kind of work. Don't be afraid to try different jets, accelerator pump cams and nozzles, and power valves in your carb. You have the instrumentation to figure out what you did wrong if it doesn't work the way you expected it to, so there is no harm done. Don't drill or file stuff unless you know exactly what you are doing, and you are prepared to buy new parts if you screw up your old ones.


I've been threatening this for a long time, and it's becoming apparent that many of you out there do not want to read, and would rather email me with questions that are already answered on this page.  So, I'm going to have to modify my rules:

I will NOT email you back if your question is already asked and answered on this page.  If you don't get a reply from me within a few days, then you need to re-read this page.  It's just getting to the point that I don't have the time or patience to type out a reply telling people to re-read the page.  About 2 of every 3 emails I get lately have been like this.  So, if you don't get an answer, re-read the page.  Also, try reading a book on Holleys.  I have a bunch for sale at my webstore, and you can get them at local bookstores and at your local public library, which are free.

I also do NOT sell parts, so please don't ask to buy any carb parts or rebuild kits.

Click Here to Email Gerard

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