Could be caused from a warped upper half of the carburetor. I once observed an internal leak at the parting line where the top of the carb meets the air horn. As I opened the throttle to raise the RPM, gas would pool up near the gasket and as I let off of the throttle, I could watch the gas evaporate before my eyes. There were not leaks on the outside.
The biggest problem with a warped Model B is not external leaks, but rather, the internal leak. These internal leaks are pulling gas away from where it belongs and putting it where it does not belong. This will cause a stumble upon acceleration.
