Is the stumble off-idle or part throttle? Could be the accel pump nozzle having the wrong size. Sounds like it's dumping fuel if it goes rich, but it also could be too small of a pump shot if it goes rich then lean. You may need to move the linkage to the top hole for more shot and a smaller nozzle to allow that shot over a longer period of time. I'll brush up in the Carter tuning manual...