I have a 1985 Yamahopper QT50. It will go from riding great and hitting 30 mph for a long ride to bogging down and only hitting 10 mph the next time I take it for a ride. There are no adjustments being made between the rides. I have cleaned the carburetor, new fuel lines and filter, and just went to premix gas and stopped using the oil pump. It tends to have an inconsistent idle and can go from high rpms to low while just sitting on the stand. I need some help, any advice would be much appreciated, thank you.