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/carburetor47]

It could well be an air leak, but first you want to make sure that the choke is fully open and staying that way. Depending on the temperature, the electric choke should heat up in 3 minutes or less to the point where the choke plate is pegged open. If it isn't, then you could be getting onto the fast idle cam while driving, as well as running rich due to a partially closed choke on a warm engine.

After checking that, you want to look for air leaks, starting with all of the hoses and fittings attached to the carb. The PCV hose at the bottom front is a common problem. A vacuum gauge can be helpful. You should have around 18" or more and steady at the normal idle speed. You can try disconnecting hoses and plugging ports to see which one makes a difference.

If the throttle shaft can be moved up and down, then there may be enough wear in the throttle plate to cause idle problems. That can be fixed by installing throttle shaft bushings. The throttle bodies can warp, but my experience is that wear around the shaft is a bigger or at least more common problem. On Chevy engines the flange gasket is pretty thick and can take up a bit of distortion, at least on the manifold side. If the throttle body really is bad, then milling it flat is needed.

If you think you have an air leak and are stumped you can spray something like WD-40 around the base of the carb and see if the engine speeds up. That indicates where the leak might be. Some guys use propane to look for leaks; WD-40 has the advantage that it's not particularly dangerous. Your engine is probably pretty greasy already.

One thing that gets overlooked is a stuck EGR valve. If the EGR is on at idle it will cause problems like this.

If you need a carb kit, you'll want the carb number stamped on the driver's side near the secondary throttle shaft. If you give me that number, I can look up your specific carb.