I have a 1968 Pontiac Firebird with a freshly rebuilt Pontiac 400 engine in it which seems to be giving the symptoms of a vacuum leak.
The car starts up, but requires warm weather and some feathering of the throttle to stay alive for approximately 5 minutes. Once the car is up to temperature, it is able to idle on its own, but I am unable to get it to idle steadily below 1000 RPM or so—the RPMs and vacuum will bounce around between 5 and 10 inches as the engine approaches the factory idle of 650RPM. The engine will often stall out, especially if I try to lower the idle speed.
This problem screams VACUUM LEAK to me, but I cannot seem to find any vacuum leaks—I have gone through hell and high water spraying soapy water all over the intake manifold/carb and do not see bubbles. I have also tried spraying carb cleaner to look for leaks and have not had any luck. I tried blowing cigar smoke through a carb port, with no leaks found.
Here's some info that may help me narrow it down:
-- Motor is freshly rebuilt, with compression in the 160-170 PSI range -- Initial timing is set to 6 degrees (factory spec) -- Dwell is set to 30 degrees (factory spec is 28-32) -- Spark plug gap is 0.035 (factory spec), and spark plugs are not fouled -- Ignition coil and condenser/points have been replaced -- Both idle mixture screws on the 750 CFM Quadrajet carb are set to 1.5 turns CCW from fully-turned-in (starting position). I do not get any meaningful response from the idle mixture screws when trying to tune them. -- Carb was used about 3 years ago for a few miles, then taken off the engine while we were rebuilding it. I took apart the carb to make sure there was no gunk/corrosion in it. Carb looks great from the inside. I sprayed it thoroughly with carb cleaner & re-assembled, and the problem persists. -- In an attempt to rule out leaks at the intake manifold, I took off the intake manifold, bought a new intake manifold gasket, and used Permatex Ultra Gray to seal it, and tightened the intake manifold bolts in the correct order with the proper torque specs. -- While looking for vacuum leaks, I have been sure to disconnect lines such as the timing advance, brake booster, etc, and plug their ports on the intake manifold/carb. No luck.
Here's a video of the engine idling, with vacuum gauge and tachometer hooked up, for visual purposes:
Thanks for posting the video. With vac that low it appears to be a big intake manifold or carb base gasket leak. An old trick often used is to spray carb cleaner around the base of the carb to see if the rpm goes up. Same can be said for the intake manifold. Be careful as it is flammable. If the valves were not set correctly [too tight] it could be a valve issue. If the heads were machined, [re-surfaced] there may be a geometry issue that prevents the manifold from sealing. BTW, since it is a vacuum, a leak would just pull the soapy water in. ala water injection Good luck...
I am no expert and not saying this will fix your problem by any means, but I would bump up the timing unless you are using a better octane gas than 92 or 93. I originally set my timing to what the book calls out and it was a no go as my engine ran crappy with the iron heads. Once I set the timing up to around 12 it ran a lot smoother and my vacuum wasn't going all over the place. I also moved dizzy vac source to manifold vac. You still might have a vac leak but this might help you idle and run smoother to figure things out. JMHO. Good luck!
I'm not an expert either, but I agree the timing should be higher. The 1968 Pontiac service manual says 9 degrees for a V8 , 6 for a six cylinder. I'd start with 12. You may have the idle screw set so high, to keep the engine idling with the low advance, that the throttle plates are out of the idle circuit and into the transition circuit. That may be why you have no change with the idle mixture screws. Been wrong before.
It took me a while, but I eventually resolved the issue.
I did eventually increase timing, to either 12 or 14 initial (can't remember off the top of my head, but total is 36 at ~2800RPM), which seemed to improve the issue a bit.
I didn't mention it in the above posts, but once in a while, the car would start up just fine, idle fine for a while, then eventually stall out. When I tried to start it up again, it would just crank forever with no actual ignition.
At some point in the time, I decided to just say "screw it", and drive the car with what I thought was a high idle. I got about 60 miles out of it, and did notice that it was getting progressively harder to start up as the miles piled on between trips.
A week later, I tried starting it up, only to completely flood the carb (gasoline coming out the vent).
Eventually traced it down to a carb float bowl which had a microscopic leak in it, and was gradually letting fuel in when it was submerged. Once I replaced the float bowl, the issue went away.
Now the car runs fine with a 750RPM idle, and I have put approximately 1400 miles on it.
Moral of the story: Never assume that a rebuilt part bought from a reputable dealer (Jegs) is functioning perfectly. And sometimes, you just need to keep the project moving forward and troubleshoot on the road.
It's possible that the issue I described in my original post was primarily timing related, but I definitely think the leaking carb float was just adding to the confusion.
Hate to bump an old post, but just in case someone with similar issues comes across this thread, I've moved the above video to here: https://vimeo.com/356801799
I was unable to edit the original post, hence the reply.
