will it boil over when you turn it off? turn the fan on and put a shop rag in front of the grill if the air will keep the rag held to the grill you are pulling enough air. check the lower rad hose to make sure it is not sucking flat.. when it is cold take the cap off put a cheap candy thermomter in the radiator crank it up to see if the thermostate is opening at the right temp.and that the gauge is reading the same temp. if all this checks good it could be timming or carb to lean.. mine was temp gauge was 40 degrees off