[stoveboltgeek]

I had this exact same thing happen to me with my '57 Chevy with a Rochester 4GC. Okay - different engine, different carb, but hear me out.

I did a fresh rebuild of a 4GC I got off eBay (replacing the Edelbrock that was on the car when I bought it). Set everything exactly to specs, including the clearly misadjusted secondary lockout (keep that in mind). Took it out and it ran great until I stomped on it and let off the accelerator - it kept going.

Fundamental to what happened is understanding the nuances of carburetors.

The choke is held open by a combination of relaxed thermostatic spring and vacuum break. When you stomp on it, vacuum drops to near zero, so the vacuum break can allow the choke to close a bit.

In my case, that's exactly what happened. The choke partially closed at full throttle, and the now-correctly adjusted secondary lockout tab jammed the secondary throttle linkage, allowing the primary to close but holding the secondary partially open.

If you road test it, a few things to try.

First, if it happens, shut off the ignition and safely coast to the side of the road without touching the accelerator. Pop off the air cleaner and see if either the secondary or primary throttle plates are stuck partially open. Then, see what's hanging them up.

Second, if you're in a situation where you can't do that, kick the accelerator pedal a few times and see if it comes down.

You can also try wiring the choke fully open and see if the problem disappears.

Based on my experience described above, I would check for a leaking vacuum break diaphragm and misadjusted secondary lockout. Some vacuum diaphragms have a controlled vacuum bleed, but if you can't pump it up with a handheld vacuum pump (Mityvac or equivalent) try a new diaprhragm.

In my case, the vacuum break operated with a brass piston instead of a diaphragm. I just re-disabled the secondary lockout like the carb's previous owner and avoided stomping on it with a cold engine.