ORIGINAL: lrb75

You may be getting pressure pulses inside the cowling. They can mess up the pump on the carb since it works by having a pressure differential from inside the engine to ambient. If ambient keeps changing it won't work right. There is a fix shown somewhere, can't remember where, to use a fitting attached to the vent hole on the carb and routinga piece of tubing into the fuse where there will be no pressure changes. Aparently this can happen to any pump type carb under the right conditions. There are many things that could have gone wrong just one more thing to check.