I suspect simple solution as well. You can isolate the fuel system easy enough by bypassing the tank altogether. Small clean tank direct to carb or small tank tubed through your pump. If your buddies think tank, I would take the tank out of the equation before you go to switch tanks. And you have cleaned all of your fuel filters including the filter at the carb?
Pump - If you didnt change anything there, I would not suspect the pump.
Are we talking - at idle in the driveway or warm and under load?
What about your timing and points? What's the state of the ignition side of the system?
We can work this out. It does not sound catastrophic.
