tank leak? fuel line? does the motor start off revving or is it about 1/2 tank in? he did say he tried a different carb with same results. i say cracked case or front bearing. take the motor out and clean it spotless on the outside, then run it a tank or so. remove tho motor and look for wet spots, start there. i just went through the same thing trying to find my backup motor for pnb3, came down to carb slider being worn and wobbly, new carb = no issues.