My 80 SR500 Yamaha has picked up a stalling phenomenon when the throttle is rolled on. It starts well and idles smoothly. Under power, the engine stalls when the throttle is rolled on, but will smooth out if the thottle is retarded. It's as if the ingnition is turned off and on, except there is never any backfire. I have taken down the carbureator and it is clean. The spark plug has been changed (no evidence of lean or rich mixture). I'm thinking something to do with timing advance mechanism or other spark control. HELP!