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C3 Vacuum leak / fast idle

1996Z15

Well-known member
Joined
Feb 20, 2001
Messages
49
Location
Little Egg Harbor, NJ
Corvette
1969 Corvette coupe
Last year I purchased a 1969 Corvette L46 coupe. One of the first things I did was to take it to Contes Corvettes in Vineland, NJ to have them go over the car and give it a tune-up.
When I got it back the guys told me that it was a very nice, unmolested car but, they could not get the idle down where it should be due to a suspected vacuum leak somewhere.
I have been told that I should take it somewhere that has a "smoke machine" so they can locate the leak and make the necessary repairs.

Is a smoke machine something that I can simulate at home; I don't know of any shops locally that would have such a machine that I can trust.
 

gmjunkie

Well-known member
Joined
Nov 11, 2004
Messages
4,547
Location
Old Fort NC/Vero Beach FL
Corvette
03 Z-51,78 L82 Coupe
Last year I purchased a 1969 Corvette L46 coupe. One of the first things I did was to take it to Contes Corvettes in Vineland, NJ to have them go over the car and give it a tune-up.
When I got it back the guys told me that it was a very nice, unmolested car but, they could not get the idle down where it should be due to a suspected vacuum leak somewhere.
I have been told that I should take it somewhere that has a "smoke machine" so they can locate the leak and make the necessary repairs.

Is a smoke machine something that I can simulate at home; I don't know of any shops locally that would have such a machine that I can trust.
Spray WD-40 on ALL vacuum hoses and fittings and around base of carb and intake with engine running!!!! When you hit the leak the engine will start running different!!!:upthumbs
 

bill81vette

Moderator
Joined
Jan 17, 2004
Messages
4,321
Location
Troy,NY
Corvette
1981 dark blue metallic
my vette has a high idle as well,and doesn't hold vacuum after I turn it off...
when I mentioned it to my buddy,who is a mechanic he suggested that I replace the check valve at the power booster as it is probably bad...haven't done it yet but I will soon,probably b4 I take it out this spring...
johnz may have another suggestion as well,He is the best!....
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
First of all, I'd find a new service shop.

With a '69, there's just not that many options as far as finding vacuum leaks. Sounds like "Contes Corvettes" is just another one of those shops which lacks diagnostic skills. Plus, ecause the headlight and the wiper door actuators are vacuum-powered, knowlege of how to troubleshoot the C3 vacuum system is a given as far as Vette Shops go.

There are only a few reasons why an engine will not idle slow enough

1) Vacuum leaks, of course. The WD40 method listed above works. What I'd do is first isolate the vacuum system by temporarily capping off all the hose connections on the carb except the vacuum advance. That will give you a better idea of where the leak is, ie: headlight/wiper door system, brake booster, PCV and etc.

Obviously, you're going to find the leak in either the C3s extensive vacuum system or you'll find the leak is specific to the engine, ie: intake manifold gaskets, carb flange gasket, leaking intake manifold fitting, etc.

If you determine it's somewhere downstream of the engine, on a car that old, if it's not been done yet, I'd simply replace the entire vacuum harness.

2) Check the ignition timing. If it's too far advanced at idle that might be contributing to the idle speed problem.

3) Inspect the carburetor to make sure it's returning to the curb idle setting and the throttle shafts are not leaking. Also check the throttle linkage.

Finding a leak big enough to make the engine not return to curb idle speed is probably not going to take a "smoke machine".
 

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