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 distributors, but I do have some helpful ideas.
I am not an expert on distributors, therefore, I am not able to accept or answer questions on ignition timing. This page is informational only. If you have more information that can be added to this page, I would welcome it.
You may have noticed when you set the ignition timing on your engine, that the idle speeds up when you advance the timing, and slows down when you retard it. This works to a point, until you get too much advance or too little advance in the system. You may have noticed that about 12 to 15 degrees of advance at idle makes the engine peppy at low rpms, but makes it ping excessively at higher rpms. Backing the timing down to 6 or 8 degrees gets rid of the ping, but low rpm response is lost.
What to do, what to do?
Some Ford point style distributors have a built-in method to fix this problem. You just have to do some investigating to find it, and read this page to change it.
The way to remedy this situation is to decrease the amount of advance the distributor's mechanical advance puts into the engine, and then give it more lead at idle, so that the total mechanical advance at high rpms remains at a point the engine can live with. This is how it works:
Condition A
If the distributor's mechanical advance provides 30 degrees of crank
advance, and you set the initial lead to 5 degrees, your engine will have
35 degrees of total advance at high rpm. Low rpm performance will
be poor, but high rpm power will be good.
Condition B
If the distributor's mechanical advance provides 30 degrees of crank
advance, and you set the initial lead to 15 degrees, your engine will have
45 degrees of total advance at high rpm. Low rpm performance will
be good, but the thing will ping like crazy, and you'll break something
eventually.
Condition C
If the distributor's mechanical advance provides 20 degrees of crank
advance, and you set the initial lead to 15 degrees, your engine will have
35 degrees of total advance at high rpm. Low rpm performance will
be good, and high rpm power will be good, and the thing won't ping.
This is what we want to provide for the engine!
How to accomplish "Condition C" on your old Ford distributor
The following photo is a late 60's Ford distributor, with the breaker plate, springs, and weights removed. Not all Ford distributors are built this way. You will need to examine your engine's distributor to check for this feature.
As you can see, the slot labeled "15L" is engaged on the limit pin. This distributor is set up to provide 30 crank degrees of mechanical advance. It is 30 degrees of crank advance, because the distributor turns at half the speed of the crank, and whatever you do with the distributor, in degrees, is doubled on the crank.
On this next photo, the little clip beneath the rotor has been removed, and the breaker cam has been lifted up and rotated 180 degrees so that the "10L" slot is engaged on the limit pin.
This distributor now provides the engine with 20 degrees of mechanical
advance at the crank.
IMPORTANT!!!
In order to perform this modification, the distributor must be removed from the engine, because moving the breaker cam 180 degrees to accomplish this task also moves the rotor 180 degrees, and the engine will be out of time. You must lift the distributor up enough to clear the cam gear, and rotate the whole rotating assembly by 180 degrees to get the rotor back to where it should be.
Once you have reset the timing limiter, reinstall the clip that holds the point cam on the shaft, the advance weights, springs, and breaker plate. Restart the engine, and give it 12-15 degrees of advance at idle, and go for a spin. If it pings at high rpm, you may need to retard the timing a bit. You also may need to lengthen the 10L slot to provide a bit more advance at high rpm, but I think you'll be quite happy with the 10L slot as is.
I did this mod on my 72 Ford 390 in an F250, to mimic the advance provided by an old factory dual point distributor, and the engine really likes it. Yours may too!
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