Classic/Antique Car Repair: vacuum leak, pontiac lemans, manifold gaskets

Question
Hello,

I had asked an earlier question concerning problems I had with an ST300 transmission downshifting issue in a 67 Pontiac Lemans with a 326.  I had followed your advice putting in some Lucas transmission "magic juice" and it didn't help.  Next morning I ran some vac line from the modulator to my lap with a vac pump and a gauge.  Took the car out and when I hand pumped the modulator up to 20 inches, the problem went away.  After 15 minutes of driving, the tranny didn't need any vacuum at all to downshift normally (but still needed it for the upshift)...I assume internal pressure has something to do with that?...anyways, now I'm pretty solid on the fact that it's the low engine vacuum that's causing the problem (runs at 11 to 12 at idle (650-700 rpm).  It's a newly rebuilt motor with the same cam I have in my 69 400 which pulls 20 inches at idle so I know it's not the cam duration.  okay, ....the main question I have now is finding what must be a vacuum leak.  I've isolated the brake booster, vac advance and vac modulator...none have ANY leaks.  the needle vibrates between 11" and 12" at idle, under load it kicks up to 20" and stays there.  I've turned the mixture screws in til it stumbles and back them back out. (2bbl Rochester -- engine is completely stock) Once I'm past the stumble, I can't get any better reading on the vacuum.  The only thing that happens is that when I pop a line off (for the vac modulator), that vac leak actually increases the idle.  I assume that means the carb screws are too rich, so I back them back down just in front of the stumble.  Spraying starting fluid around the carb and manifold gaskets has no effect, and I was suprised that spraying the starting fluid directly into the carb actually kills the rpm.  That I'm not understanding at all, since shouldn't the idle kick up if the ether is pulled into the vac leak?  As far as the leak, the only thing I can't see or check is the possibility of worn valve guides that's come up in my research.  The heads were done by a trustworthy (as far as I know) shop so I don't think this could be the case (and I didn't have the vac problem before the rebuild so it shouldn't really matter)  I'm at a loss on this.  Any suggestions on what i'm missing here?  Sorry for being so wordy with this, but I'm trying to cover all angles for the near impossible task of diagnosing something you can't see.  Thanks,
Dennis

Answer
The following is a sort of "stream of thought" of things that pop into my mind as I read your story:

One thing that strikes me as quite odd is that the vacuum actually increases under load!  This is not normal operation - opening the throttle should make the vacuum drop.   I'm thinking that you might be measuring the ported vacuum, not the intake manifold vacuum.  Try measuring the vacuum on one of the lines that is connected directly into the intake manifold - perhaps the power brake booster line if you can put a tee into that, or the PCV line - something that you know is pure intake manifold vacuum.

If you're measuring the vacuum at the carburetor fitting - check to see how high the fitting is on the carburetor - if it is above the moutning flange, it is probably internally connected to the port above the throttle butterfly; that fitting belongs connected to your vacuum advance unit on your distributor, it is not intake manifold vacuum.

If the vacuum at idle is truly what you measured, I begin to suspect cam timing - especially since it perks up to normal above idle.

From what you say, I'm wondering if the cam is non-stock - I know you aren't having this problem with your other car, but perhaps its cam is timed differently?

So far, I really can't help you much - it is time to find someone with an engine analyzer to diagnose this engine - that low vacuum reading at idle is not normal!

The way to check for a vacuum leak is to let the engine idle, and take a piece of cardboard or something similar, and very slowly slide it over the primary carburetor air intake (air cleaner off). If at ANY time, you hear the idle speed increase, you know the idle mixture is too lean - and this is almost certainly caused by a vacuum leak.   Your mixture screws should be set around 1 1/2 turns out from bottoming.  Try setting them there to see what you get.

I don't understand why the shifting is changing as the transmission warms up either, but that seems to be an independent problem.

Let me know what you find out!

Dick