I recently got an onan b48 engine, basically a barn find. I don't know any history of the motor. I did the usual things: plugs, plug wires, coil, points, condenser, carb rebuild, compression check 120-125psi both cylinders. I've got it running good but its slow. It cranks right up and idles at 1300 and will rev up to about 2800 smoothly but gets bumpy faster than that. The engine plate says 18hp at 3600 rpm but I can only get it to run at about 3100 and as soon as it has a load on it, it slows to 2400 rpm. The linkages seem to be tight and free, and the throttle linkage is pulled against the stop. Any suggestions? Shouldn't the governor keep it close to top speed unless overloaded?