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Guys,

I have a 69 350/350HP corvette I'm restoring and I swapped out the camshaft for a mild hydraulic roller when I rebuilt the engine. Otherwise it is totally stock including the original Q-jet carb. I'm trying to set the timing but don't know where to set it since I swapped the cam out. The guy who rebuilt/restored the carb said I need to use a vacuum gauge to do it but I've looked at some YouTube videos and they're not very clear on how to do it. Has anyone done this before?

Mike
 

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You want to get the highest possible vacuum which means the least throttle opening consistent with a smooth transition into the throttle. There are two interdependent steps between ignition advance and throttle position for idle air flow. Then there is idle screw position back and forth to balance out the fuel flow, here that doesn’t mean that both idle screws have the same number of turns from closed as the needles and their seats have manufacturing tolerances which may not equal same/similar fuel flows plus with a dual plane intake the high and low sides are likely to have a preference.

Bogie
 

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Set your timing first with a timing light. You really want the springs in the distributor set for the proper curve. Timing should be set so it's maxed out at 32-36 degrees at 3k RPM with the vacuum advance plugged. Normally you have to play with the springs, with factory HEI one medium and one lite would be a good guess to start. At idle it should be around 12-16 with the vacuum advance plugged but worry about setting the max advance more than base timing.

Once the timing is working as it should, use the vacuum gauge as stated above to set the idle mixture. It's a bit of work, but one of the most worth while things you can do.
 

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The total timing doesn't need to change. You'll nearly always want 34-36 degrees at full advance. If your original initial advance was 12, the dizzy is trimmed to give you (let's say) 22 degrees of advance to get the 34 total. If you find that you need 16 initial, you need to recurve using weights/springs to limit that advance to 18 degrees to get the same 34 total.

Finding initial timing is not tough as it is remarkably non-critical. You want to get max vacuum by advancing the idle timing while closing down the throttle adjustment. Check your timing with a light and record what you have as initial. Now go through some testing. Light-throttle driving is fine, but don't go nuts because your advance curve will kick in around 3000 rpms and probably advance too much. Get it good and hot and try restarting it. Getting kickback from the starter? Retard timing a couple degrees. Getting detonation off-idle? Retard a couple degrees. Find the sweet spot, write down the number. Let's say it's 16 degrees. Recurve the distributor for 18 degrees of advance.

BTW... rule of thumb is that if you're getting any audible ping, you retard until you don't hear it anymore, then retard 1-2 more degrees because detonation can still be happening even if you can't hear it.

You may also benefit from an adjustable vacuum advance canister on the dizzy. I doubt you changed your vacuum potential much, but the adjustable can will help you fine tune it more. Sometimes when you advance the initial timing you get part throttle detonation on a factory vac can because it's adding timing too soon. The factory can is a fixed-pressure thing and since you've changed initial timing and potentially changed how the engine makes vacuum, you might find it a helpful tuning tool.
 

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Your given good advice. But you might find on today’s pump gas that a bit lesser total timing will work in a safer zone. A lot of near stock SBC will run good at 30 degrees total. Mix in some higher octane fuel and the full max advance will be great. If your quench area is tight it will be less prone to ping. Generally engine builders do not use thin steel shim gaskets, making quench bigger than original stock lowering cylinder pressure. With the old style “fuelie heads” large quench and resulting ping becomes a problem.
 
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