Vacuum Leak?

[Eugene V.]

Hello everyone,
I have a 1970 LS5 with the correct Q-Jet . The problem is twofold: one, it will start, run briefly stall and then will run on high idle after two or three attempts. Two after it is at operating temperature the idle speed will slowly vary between 750 and 1100 rpm. I at first suspected a vacuum leak.
I then restricted the flow into the carb and noted an increase in rpm of 400 rpm.
I then used carb cleaner to troubleshoot the vacuum leak. ( all vacuum operated devices function normally)
This action produced no results. I then disconnected and plugged the line at rear of the manifold and restricted the carb again with the same 400 rpm increase. I then repeated with the carb cleaner around the base of the carb and all around the intake manifold. I again saw no increase in rpm.
Is it possible to have an internal vacuum leak? If so can it be detected and confirmed without removing the intake manifold?
Should repacement of the intake manifold gaskets be nessecary, where is the best place to purchase the correct replacement gaskets?
Thanks for your help,
Gene Ventura

[Timothy B.]

Re: Vacuum Leak?

Eugene,

The first problem with starting then stalling could be the gasoline evaporating from the carburetor after sitting for a few days. The problem could also be the choke adjustment, after depressing the acellerator pedal the choke should close tightly. After inital start up the choke blade should open initally with the help of the choke pull off approx 1/4".

The idle surge could be the emulsion screws not adjusted properly or the primary throttle shaft has wore and vacuum leak internal especially if it's a q-jet carby.

If you have a service manial for the car double check the choke adjustment and A/F emulsion screw adjustment procedure.

[Eugene V.]

Re: Vacuum Leak?

Timothy,
Thank you for your reply.
What I have found particularly frustrating about this problem is that I have tried all of your recommendations all to no avail. I have had bushings installed in the carb base and have the choke set as specified. The emulusion screws have been set according to the manual. Adjusting these screws seems to have little effect on idle performance this is why I suspect a vacuum leak.
Thanks again for your help,
Gene

[Joseph K.]

Re: Vacuum Leak?

Just a guess... leak in the vacume advance?

[Eugene V.]

Re: Vacuum Leak?

Joseph,
I didn't think about that one. It is a new unit, but that is worth a try.
Thanks,
Gene

[Terry M.]

Re: Vacuum Leak?

Gene,

I trust you have capped off the vacuum fitting at the intake that goes to the headlights and HVAC system. Each of those can present significant opportunities for vacuum leaks.

[Edward J.]

Re: Vacuum Leak?

As Terry says, on corvettes with p/brakes, vaccum headlamps and wiper doors, I like to start by shuting the vacuum off to each one of these one at a time to pin point vacuum leak area. another test that works is spray a small amount of carb. cleaner at the base of carb.and intake manifold gasket areas to see if you can get the engine RPMS to increase this is a sign of a leak.

If you have done any work on car that you may have disturbed any vaccum hoses or left something unpluged, just do a recheck.

[Eugene V.]

Re: Vacuum Leak?

Terry and Edward,
Thanks for your replys.
I did isolate the vacuum system from the vacuum source by removing the hose and line from the fitting on the rear of the manifold and using a rubber plug to seal the fitting. I did this after using carb cleaner as described by Edward and also using the carb cleaner to detect leaks at all accessible points of the vacuum system. I.e. where the hoses connect to the headlight actuators, wiper actuator, etc. I have not attempted to individually isolate each service yet because when I did plug the source fitting the problem remained. In other words, when I restricted the airflow into the carb, with the vacuum system disconnected and the fitting plugged, the rpm increased by about 400. I then sprayed the carb cleaner at all points on the manifold where any potential leaks would be. There was no increase in rpm. I. concluded that there was no external leak at the manifold.
The engine is completely stock, and as best that I can determine, appears to have not been taken apart. For this reason, before I do replace manifold gaskets, I want to be certain that an internal leak is possible and in this case probable.
Thanks to everyone for your help,
Gene

[Patrick H.]

Re: Vacuum Leak?

I don't suppose that there is an uncapped vacuum fitting on the carb?
Just trying to think of other possible small items before you take the intake off.

[Edward J.]

Re: Vacuum Leak?

Terry and Edward,
Thanks for your replys.
I did isolate the vacuum system from the vacuum source by removing the hose and line from the fitting on the rear of the manifold and using a rubber plug to seal the fitting. I did this after using carb cleaner as described by Edward and also using the carb cleaner to detect leaks at all accessible points of the vacuum system. I.e. where the hoses connect to the headlight actuators, wiper actuator, etc. I have not attempted to individually isolate each service yet because when I did plug the source fitting the problem remained. In other words, when I restricted the airflow into the carb, with the vacuum system disconnected and the fitting plugged, the rpm increased by about 400. I then sprayed the carb cleaner at all points on the manifold where any potential leaks would be. There was no increase in rpm. I. concluded that there was no external leak at the manifold.
The engine is completely stock, and as best that I can determine, appears to have not been taken apart. For this reason, before I do replace manifold gaskets, I want to be certain that an internal leak is possible and in this case probable.
Thanks to everyone for your help,
Gene

Eugene, if you suspect a internal vacuum leak you can remove the the oil cap/ pcv and spray a small amount of carb cleaner into the inside of engine and RPMS will increase if theres a leak.

[Eugene V.]

Re: Vacuum Leak?

Ed,
Thanks for your reply.
I believe that I have found the problem. I turns out that the carb was running lean at idle. I did some adjusting and the problem appears to be corrected. When I partially cover the air horn the idle rpm no longer increases. I hope to have it out for a test ride over the weekend.
Thanks to everyone for their help,

Gene

[Keith L.]

Re: Vacuum Leak?

Please explane what partially covering the air horn and rpm increasing would mean?

[Eugene V.]

Re: Vacuum Leak?

Keith,
By partially covering the air horn one is performing the same function as a choke plate: reducing the amount of air entering the engine. If the air/fuel ratio is correct, the engine rpm should decrease, with the engine wanting to shut down. On the other hand, if the rpm increases, the the air/fuel ratio is incorrect. The two possibilities are too much air (vacuum leak) or to little idle fuel. in my case the problem was too little idle fuel. This resulted from an incorrect part installed during the rebuild.

I hope that this answers your question,

Gene

[Keith L.]

Re: Vacuum Leak?

Thanks, I remember a friend doing this before. Sometimes I forget what I remember and sometimes I remember what I forgot.