I have the same problem. Mine still has the pump but it runs like crap. I have heard that the diaphragm in the pump will start leaking after a few years and that's why it won't run right. But, I tried bypassing the pump by connecting the tank directly to the carb. Muffler pressure to the tank just like any other glow engine. Runs a lot better! I am hoping that as long as the tank is above the carb it will work. Putting it inverted in a Top Flight Mustang, so we shall see.