I was thinking that after you changed the pump the needle valve in the carb was sticking open. What you just told me, I was wrong. Now I'm leaning toward a blown head gasket. While running at mid throttle, if you loosen the dipstick cap and let it rest on the fill tube, does it bounce up and down. If it does, that means you are getting excess pressure in the crankcase and I would look into a bad head gasket. Do you have a compression gage or cylinder leak down tester? Also, please post your model numbers.