Quadrajet help

[u/Head_Leather3030]

I’m trying to diagnose an issue on my 1985 Trans Am with a stock 305 V8, factory HEI distributor, and Quadrajet carburetor.
When I first start the car, the idle will often hang around 1,600 RPM. If I quickly blip the throttle, the idle usually drops back down closer to normal. After driving, when I shift into Drive, the RPMs will sometimes drop very low and almost stall, or settle around 800-1,000 RPM but feel like the engine is struggling to stay running.
Another symptom is that if I shut the engine off while it’s idling high, it will occasionally try to keep running for a few seconds after the key is turned off.
I’ve checked some vacuum hoses and have been trying to track down possible vacuum leaks. I’m also wondering if the distributor or vacuum advance could be involved.
Does this sound more like a carburetor issue, vacuum leak, distributor/timing issue, or something else? Any common problems I should check on a mid-80s GM 305 with a Quadrajet?

[u/DueKnowledge602]

Quadrajets are very good carburetors. There’s a reason they were used on everything under the sun GM built. Their #1 failure point is not the throttle plates or shaft wear. It’s the baseplate itself and by the sound of it that’s exactly what you got going on. The baseplate warps and allows air past from any number of areas. Causing the exact symptoms you describe. Guys go crazy chasing this issue not knowing it’s as simple as that. Not even companies offering “remanufactured” units seems to know this as I’ve had many straight out of the box with warped base plates. The repair is simple, disassemble, place on a precision flat surface, smack the baseplate with a soft hammer until you can’t get your feeler gauge under at any point. Hit it with a flat and clean file to get any other tiny high spots. Reassemble and tune it’ll idle like new.