The fuel pump's job is to keep the bowl full. However, it is actually air rushing through the carb that creates a venturi effect. The main metering jet determines how much gas is injected into the air stream. I'm not sure vacuum has anything to do with your problem. It could very well be crap in your main jet. I would take the air cleaner off, rev her up, and put your hand over the carb. Quite often this will suck the trash through the jet and it will run better.
