Miles, my truck, has had this problem since I got him, where he'd start, warm up, and then die when I let off the gas. Me and a couple friends on Discord thought it could be a dirty carb, a vacuum leak, or bad tuning. I've run up and down his ignition, his afm, and his idle screw, but nothing changes. He'll start cold, like if he was fuel injected; he idles fine, if not a little high, revs up and down, but then he gets up to temperature, and then refuses to idle.

I can keep the engine running with my foot on the gas, but the engine dies as soon as I let off. His bowl stays at the level so I know it's not the fuel pump. He revs high and low warming up, so I know it's not ignition. I've accounted for every vacuum line and plugged the ones I'm not using, so I'm pretty sure it's not a leak. I've tuned the carb to hell and back, to no avail. I've triple-checked that the camshaft is in time, and it is. No matter what, he just refuses to idle once he's warmed up.

 

Maybe a bad accelerator pump in the carb. When you initially step on the gas, the accelerator pump is supposed to squirt a shot of gas in immediately to match the increased airflow of opening the throttle. After the initial squirt the carbs metering catches up and continues to supply the increased fuel required.

A bad accelerator pump might cause a substantial lean condition right at the point you step on the gas. That lean condition can be bad enough to make the engine stall.

Here is what to check. After the engine has been idling, shut it off. Remove the air clearer lid and look down the throat of the carb while opening the throttle by hand. If the accelerator pump is good you should see a nice little stream of gas squirting in. Remember, do this with engine off

 

Nah, he revs up perfectly fine, just dies coming down. My bad, I should've clarified.

 

Turns out, last time I built the carb, I let the ratchet piece that holds the choke shut unwind off its spring. It's like one of those magician puzzles in there, so another rebuild is in store! Here's to doing it right the second time!