i have been having the same problem, sometimes my bike starts up fine no problem and sometimes it will start right away but as warming up it will start to bog down, no matter if i try to rev mor rpm. once it dies it wont start no matter what i do. Carburetor is getting gas, there's spark and good compression, Bike starts if i prime the carbs. Mechanic told me it was starving for fuel because my valves are too tight. Valves was readjusted and so far my bike ran a lot lot better(Faster) and not to mention starts up fine no problem anymore cross my fingers. So hope this helps.