i might be time to take the motor out and blow it out. make sure you note the timing position and take your time. I had the same issue once, I took the motor apart and cleaned it, put it back together and made sure everything was tight. and it worked fine. also, make sure nothing is loose. I had a motor once get a loose timing adjuster and it was at like -120 deg timing and it didnt work from crap. I readjusted the timing and it was good.