Can't think of anything else because this is the only thing that makes it do it. I've been through the carburetor enough to do it with my eyes closed, every jet, air vent, bleed, and seal is good and clean, in good shape. The fuel pump pumps a tremendous amount of fuel. Three or four seconds of cranking and there is almost an inch or so of fuel in a pop bottle. Good fresh fuel, she drives it all over the lawn no problems it always fires up and runs awesome. It only does it when hooked to the sled.