Hi there,

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:



If this is not a vacuum leak, what could it be?