72 vacuum troubleshooting

[Gary S.]

re: my earlier post on spark plugs.

I did a quick measurement of the vacuum on my 72 LT1 with original cam, pistons, etc. With the engine warm and the vacuum canister hose attached, I get 15" hg. This is a steady state reading with no fluctuation. I would have expected 17-19" but I only have experience with SB Chevy's from this 72 and an earlier 66 L79. Is this low/high/normal?

I will check the vacuum at the port at the back of the manifold later tomorrow, and then with that port plugged. I bought a vacuum troubleshooting guide and will go through that later on.
Gary

[Duke W.]

Re: 72 vacuum troubleshooting

Vacuum is dependent on idle speed. Idle vacuum at 900 will be higher than 750, so what idle speed do you achieve this vacuum? you need to quote vacuum readings at a given idle speed.

Also the way the engine is tuned (particularly total idle timing) make a difference. Full vacuum advance at a given idle speed will make more vacuum than ported vacuum advance.

You can have good idle vacuum, but still have a leak which leans the mixture. Check all your vacuum hoses and vacuum devices. Also the manifold runner seals and carburetor gasket. Spraying carb cleaner or mineral spirits on suspect vacuum leaks can help. The momentarily richer mixture will usually cause cause a brief increase in idle speed.

On a 327 the Duntov and LT-1 cams should pull about 12" at 900 with 25-30 degrees of total idle timing; 10" under the same conditions with the 30-30 cam, and 14" for the L-79 cam. The 300 HP cam should pull about 18" at 500 with 20-25 degrees of total idle timing.

Duke