Because the throttle is mostly closed, very little air is moving into the cylinders.

This is because the throttle is mostly closed and the engine is pulling against this restriction. As you are probably aware, at part throttle the engine will create a certain amount of vacuum in the intake manifold. This establishes the amount of timing the engine will see at wide open throttle (WOT). This is determined by the weights and springs spinning around inside the distributor. These are typical numbers.įirst, let’s look at the mechanical advance portion of the timing-the 34 degrees total. Let’s say it now reads 49 degrees, which would mean the vacuum advance is worth another 15 degrees. This number will now be greater than 34 degrees with the addition of the vacuum advance. Now connect the vacuum advance from the carburetor to the distributor and read the timing on the harmonic balancer while revving the engine to 2,800 rpm. With 34 degrees of total mechanical advance and 14 degrees initial timing, you have 20 degrees of mechanical advance-14 + 20 = 34. Ideally, the timing now should be somewhere around 34 to 36 degrees to total advance. Make sure the vacuum advance connection is removed, and now rev the engine up to around 2,500 to 2,800 rpm. This can be checked with the engine running at idle with a timing light. The better plan is to start with a decent initial timing of roughly 10 to 14 degrees. You can get around this with an ignition cut-out so that the engine cranks and then you hit the ignition circuit, but this is unnecessarily complex for a street car. But with this much initial timing in the engine it will be difficult for the starter motor to crank the engine. If we had to use a generic maximum advance that seems to work for most pump gas engines, somewhere around 34 to 36 degrees before top dead center (BTDC) is a good number. First, on a street-driven engine there is no good reason to lock out your ignition timing. Jeff Smith: You’ve addressed several issues, but they are all related to ignition timing. What kind of timing should I be running and should I disconnect the vacuum advance? Right now the engine has an Edelbrock Performer intake, a 600 cfm Holley four barrel carb, and an HEI distributor. It’s a nice little cruiser and not really a hot performance engine.

Is this a better way to go with a street engine? I have a small-block 350 Chevy in a ’67 Chevy pickup with a TH350 trans and 3.73:1 gears. In fact, I’ve heard guys say they’ve locked out their advance and just run a fixed timing amount. I’ve always thought that vacuum advance was something just for stock engines and something you didn’t use with a performance engine.

Delco vacuum pump in big cam 350