I don't know about modifying the YF, but swapping on another carb would be where I'd start in order to eliminate the carb being the problem .... like Roy, I'm also puzzled how enough gas could be getting in the crankcase as to make a real difference in volume and viscosity, if via the carb I'd expect it to run poorly under any condition, if via the fuel pump I'd think the pump couldn't keep up to the carb at more than idle, particularly under load, although I guess you haven't been really loading it
Bill
