Motorcycle Questions? Ask a Mechanic.
Hi, could be the fuel tank cap air vent is plugged or your fuel valve on the tank may be failing. The fuel valves are often opened by engine vacuum and can fail to allow fuel to flow at higher rpms.
Check your spark plug color which can tell you what is happening inside the engine. Black is too much fuel or not enough air flow,
whitish is the fuel is lean or you have intake air leaks.
A light brown or tan color is best. Sometimes even new carbs will have sticky floats inside which stop the fuel after a few minutes of running. Cleaning the float needle valve can help and checking the floats operation. Let me know what else you need to know.