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The leak can be underneath....where you cannot see it......remember......most of the port is in the valley pan. Vacuum gauge is the tell
Jebby
Got the basic vette no power anything. one vacuum line to manifold which I have plugged in addition to carb vacuum hose. with no change. placing my finger over PCV valve as the final plug off area RPM drop down with a decrease in vacuum to 5.
From: At my Bar drinking and wrenching in Lafayette Colorado
Holy crap - it's idling at 5" of vacuum..? Yeah, that's a problem.... I think funny cars idle with better vacuum than that... How much initial timing are you running?
Holy crap - it's idling at 5" of vacuum..? Yeah, that's a problem.... I think funny cars idle with better vacuum than that... How much initial timing are you running?
From: At my Bar drinking and wrenching in Lafayette Colorado
If you have a vacuum leak, plugging all hoses will cause rpm to drop.
You don't have enough timing. Bump your initial timing up to 16-18. Suck on your vacuum advance hose and verify it's not leaking and that the vacuum advance is moving. Then hook up vacuum advance to manifold vacuum to add in about 16 degrees (should be) of additional timing so that you have about 32-34 degrees of timing at idle. See what that does...
If you have a vacuum leak, plugging all hoses will cause rpm to drop.
You don't have enough timing. Bump your initial timing up to 16-18. Suck on your vacuum advance hose and verify it's not leaking and that the vacuum advance is moving. Then hook up vacuum advance to manifold vacuum to add in about 16 degrees (should be) of additional timing so that you have about 32-34 degrees of timing at idle. See what that does...
vacuum advance moving fine no hose leak. Advanced timing to 18, a little easier to keep rpms at 750 but slight turn on idle screw and it drops to 600 or up to 1000, vacuum now reads 11. Connecting vacuum advance to manifold vacuum RPMs advance to 1200 and timing mark moves up where it cant be seen (dont have a degreed wheel or degreed timing light to know actual timing at this point), Drove car, breaks up/ bogs under passing acceleration returns to normal letting off gas.
Look at your timing marks on your balancer. Measure the distance between 0 and 15. Now take the tape do it again from 15 out that same distance and put a white Mark with like white out or something. Now you have a 30 degree mark.
Last edited by derekderek; Sep 2, 2018 at 08:41 PM.
so you will have two or three marks on your balancer. The original one which whenever you start seeing 15 or 16 degrees on the timing light then the second one will start showing up around zero. Then with centrifugal and vacuum advance kicking in you can see where your timing is going to instead of just guessing it's over there somewhere.
Last edited by derekderek; Sep 2, 2018 at 08:44 PM.
From: At my Bar drinking and wrenching in Lafayette Colorado
Look down the venturi of the carb with a flashlight while the engine is running and see if there is fuel dribbling out of the main discharge nozzles at idle (the center of the booster venturi).
Look down the venturi of the carb with a flashlight while the engine is running and see if there is fuel dribbling out of the main discharge nozzles at idle (the center of the booster venturi).
vacuum advance moving fine no hose leak. Advanced timing to 18, a little easier to keep rpms at 750 but slight turn on idle screw and it drops to 600 or up to 1000, vacuum now reads 11. Connecting vacuum advance to manifold vacuum RPMs advance to 1200 and timing mark moves up where it cant be seen (dont have a degreed wheel or degreed timing light to know actual timing at this point), Drove car, breaks up/ bogs under passing acceleration returns to normal letting off gas.
If this is a Quadrajet, with air cleaner off, push open the large secondary air flow flaps at the top of the carb. Make sure the hanger for the secondary enrichment needles is raising when the flaps are opened. Sometimes the fiber "cam" falls apart due to age. When this happens, it can cause a lean condition because when you go full throttle, the secondaries open, but the needles don't raise out of the jets so there is no additional fuel to go with the extra air. Also, If your fuel pressure is truly 2 PSI, it is too low. Should be at least 5 PSI.
If this is a Quadrajet, with air cleaner off, push open the large secondary air flow flaps at the top of the carb. Make sure the hanger for the secondary enrichment needles is raising when the flaps are opened. Sometimes the fiber "cam" falls apart due to age. When this happens, it can cause a lean condition because when you go full throttle, the secondaries open, but the needles don't raise out of the jets so there is no additional fuel to go with the extra air. Also, If your fuel pressure is truly 2 PSI, it is too low. Should be at least 5 PSI.
Thanks for the idea on this.The carb was rebuilt not long ago so everything is pretty new. I did check this anyway as you suggested and it all seems to be working fine. I'm stumped.
I believe I solved some of the issues. Turns out I left an old electric fuel pump in place when i resurrected the car after years of storage. My brother installed it years ago when he used to run a Rochester fulie unit on the car. I should have known to replace or remove it. That was the last thing I checked and it seems it was restricting the fuel flow intermittently. once I bypassed the pump the car has been much more reliable. my idle speed still bounces around between 600 and 750 rpm and still runs rich.
Last edited by greggome; May 30, 2021 at 11:29 AM.
You might try posting a few close up pictures of your carburetor. It should be a Quadrajet, but heaven knows after all these years how many times it has been into or even replaced. With good pictures, someone here may well see something that provides another lead on resolving the issue.
Lars has more than once told stories about the hazards of buying or installing commercially rebuilt Quadrajets, if that's what is currently on your car.
A picture with these numbers would be particularly useful...
This chart may help you to decode the number you find...
its the original since we have had the car since new. never been rebuilt except 2 years ago. was running a holley for years then had this rebuilt due to the issue mentioned
Last edited by greggome; May 30, 2021 at 04:56 PM.
