Hi I thought it could be the headgasket. I own a 150 honda carb model. the haed was milled and a reprifield cam was added , and a weber 36DCD carb with couls air intake was also added. it started a while agao to run warm when doing WOT on the high way. But now it ocours more often. When driving in town the heat seems fine and it doesnt overheat, but as soon as yoy reach the highway and go all out it tends to overheat, but by putting on the heater the temp seems to drob. had a look in the overflow bottle and it was full and overflowing and rad, needed topping up, so every one suggested, that the headgasket was shot. Well i emptied the overflow bottle and went for a drive, bottle didnt filled up or over flowed, didnt top up the radiator. When i topped it up the rad, the bottle was again filled over its max. level after a hard drive. Could it be a blocked rad or is it the head gasket? termostate was replaced and the cap and all hoses.
Goint ot to a pressure test today
Need help please

 

does it smoke? let it warm up then pull one of ur plugs to check for moisture=head gasket, try a leakdown or compression test

 

Definitely headgasket...