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5150.4.67 · 06-07-2011, 01:29 AM

Dizzy timing is something of an art. Especially with cams with alot of overlap.
It all depends on the engine and how good it runs at idle and across the rpms.

Some engines run much better at idle with a boat load of advance. This helps it burn cleaner AND cooler.
Lets say something like 26 initial degrees makes it idle just like you want (some can go as far as 50!).
Now, you can't set the dizzy to 26 degrees initial timing because when it reaches its full advance (20 degrees more) at around say 4000rpm, you will be at 46 degrees of advance! Way too much.

So now you set your initial timing to 16 degrees with the added 20 degrees of mechanical and you have the magic number... 36 degrees total advance! But now it won't idle worth a crap.

Enter Mr. Vacuum advance. He can add 10 degrees of advance to the party. Hooked up to manifold vacuum it advances the timing another 10 degrees, but only at idle, cause when you hit the go pedal the vacuum advance disappears and the mecahnical takes over! And now it idles again because you have 26 degrees of advance at idle!

Moral of the story is each dizzy needs to be set up for the engine combination its going in. Manifold vacuum or port vacuum? # of degrees of mechanical advance? Initial timing to idle right? ..............

All the numbers above are variable except the 36 total degrees of advance WOT. That number doesn't change much.

So to just drop in a distributor out the box 9 times out of 10 doesn't have the right mix unless its bone stock and meant for that setup. That's usually why so many folks have problems with their carb and can't get it to run right. Its usually not the carb!

Anyway my $.02

Wow, did that make any sense?

06-07-2011, 01:29 AM	#11
5150.4.67 Registered User Join Date: Dec 2007 Location: Modesto, Ca Posts: 411	Re: Thumpr & timing oddity... Dizzy timing is something of an art. Especially with cams with alot of overlap. It all depends on the engine and how good it runs at idle and across the rpms. Some engines run much better at idle with a boat load of advance. This helps it burn cleaner AND cooler. Lets say something like 26 initial degrees makes it idle just like you want (some can go as far as 50!). Now, you can't set the dizzy to 26 degrees initial timing because when it reaches its full advance (20 degrees more) at around say 4000rpm, you will be at 46 degrees of advance! Way too much. So now you set your initial timing to 16 degrees with the added 20 degrees of mechanical and you have the magic number... 36 degrees total advance! But now it won't idle worth a crap. Enter Mr. Vacuum advance. He can add 10 degrees of advance to the party. Hooked up to manifold vacuum it advances the timing another 10 degrees, but only at idle, cause when you hit the go pedal the vacuum advance disappears and the mecahnical takes over! And now it idles again because you have 26 degrees of advance at idle! Moral of the story is each dizzy needs to be set up for the engine combination its going in. Manifold vacuum or port vacuum? # of degrees of mechanical advance? Initial timing to idle right? .............. All the numbers above are variable except the 36 total degrees of advance WOT. That number doesn't change much. So to just drop in a distributor out the box 9 times out of 10 doesn't have the right mix unless its bone stock and meant for that setup. That's usually why so many folks have problems with their carb and can't get it to run right. Its usually not the carb! Anyway my $.02 Wow, did that make any sense?