Starts fine and runs great till you hit 3500 rpm, at that point motor sounds like it's running on 5 cylinders, no backfiring. will not increase rpm under load. You back off throttle and it's running smooth again. It will run all day long at 3400 rpm
Found problem, was the shift stall switch hanging up. When you push shifter to wide open throttle, the linkage was hanging up and engaging the stall switch.