(I posted this over on TBN, but only got one reply. Sorry if you've already seen it.)

This one's got me baffled.

The engine is a Briggs & Stratton, 4 HP, made in the early 60's. It will start on full choke, first pull. If I move the throttle off choke, it wants to die. If I'm fast enough, and choke it again, it will continue to run. It won't keep running on full choke, though. Normally, I would guess some type of air leak. But here's the thing that doesn't really fit that guess: if I move the throttle off full choke, to near idle (immediately after starting it) it runs quite nicely. It seems like it could idle that way until it ran out of gas. But if I push the throttle forward, and the RPMs go up, it will quit. (Unless I choke it again.) I think the carb is a vacu-jet. I have replaced the small rubber gasket that makes up the valves and pump, and cleaned it thoroughly.

Any ideas?

Thanks in advance. Bret