I feel kind of ridiculous posting this since there are COUNTLESS threads about vacuum leaks already, but I'm at a total loss and need some creative ideas, so here goes.

1979 L-82, stock quadrajet, intake, and stock engine as far as i can tell. No performance upgrades, emissions equipment has long been removed.

I basically haven't been able to get this car to consistently idle nicely for as long as I've had it. I just rebuilt the q-jet.

When I first removed the q-jet it looked like the base gasket had failed, there was carbon buildup and a little bit of corrosion between the carb and the intake, cleaned them both up pretty good and put in a new gasket (I was pretty confident that this was going to solve my problems, but alas).

There was a broken primary metering rod in the carb, replaced that.

The air horn is pretty severely warped from over-torqueing, however the gasket had well-defined sealing ridges indented into it so I decided to leave it alone.

Idle mixture screws set to 3 turns out to start with, timing set to 12 deg BTDC, mechanical advance behaves how it should.

When I first put the carb back on, it started right up, idled and ran BEAUTIFULLY, drove it around for a while and after parking it, running into a store and back, it was back to poor idle, dying at every stoplight.

Cupping my hand over the primary side raises the rpms to normal (which I believe is tell-tale sign of vacuum leak).

I've disconnected and capped every line from the carb and intake, no change. I've sprayed both propane and carb cleaner around the base of the carb and the intake mating surface, nothing. I've checked the vacuum advance, behaves how it should. Checked PCV, also good. EGR valve is non-existent and theres a plate blocking off the port (also sprayed around that, nothing). I've tried blowing smoke through the system, no signs of leakage with that either.

I'm thinking the next logical step is to just replace the intake manifold gasket even though I can't find evidence of a leak there. Anyone have any other suggestions? Things I may have missed or solutions to similar problems you've experienced?

 

after you turned the mixture screws 3 turns out and set the initial timing with vacuum advance disconnected, did you adjust the mixture screws to best lean idle at the correct idle speed?
what did you set the float level to, 1/4" ?

 

Sorry, I should have clarified the order of events. I had set the timing a while before the carb rebuild. Post-rebuild I set the mixture screws to 3 turns out as an initial setting with the intention of doing a final tune after driving it for the day (getting it up to temp and all that). But when I got back and it wasn't staying running, leaning out the idle mixture only made the issue worse. I spent a little while attempting to tune it but it was clear that trying to tune it in it's current state wasn't going to get me anywhere based on erratic and very low vacuum readings. Idle WAS running steady at ~900 in park, ~650 in gear.

I double checked the timing and it is still where I set it, 12deg with vacuum advance disconnected.

Float is set to 15/32" per the spec on the rebuild sheet.

 

All I can think is that or choke wasnt fully open or the high idle was stuck when yo first adjusted it and then release itself or something along those line. I would start over with the carb adjustments and see if redoing them has any effect. I would suggesst maybe your distirbutor advance was sticking but it seems you checked that as well

 

I did check that the choke was operating correctly before taking it out after first installing the carb. Fast idle kicked down and choke was fully opened. I checked with a timing light and the advance didn't seem to be sticking and the mechanical advance weights move freely.

 

15/32 is almost a half inch, any performance rebuild states 1/4", the float level is too low. Try raising it by setting to 1/4".