1973 65hp 650 mercury problem

Just bought a 1973 aluminum boat with a 650 mercury motor dunno exactly what the year of the engine is but pry about the same. The guy started it up and it seems to idle and start fine.......he said that it will run fine up till the boat is about to plane then start running rough and will not run good. he replaced the spark plug and said it did not fix it. he showed me that when he pumps the primer thats like a ball on the gas line to get fuel in when your about to start that the carburetor starts leaking gas. Im just wondering if this is probably the problem with the rough running after 10 mph and what i can do to fix this. any other advice on what i could do to tune it up since i just bought it would be greatly appcreciated. my uncle is a retired mechanic and im gonna take it down to him tomarrow cause i really know nothing about these kind of engines.


thanks

Re: 70's 650 mercury problem

You need a thorough carb cleaning. The inlet needle is not properly sealing against the inlet seat which is why you can pump gas out of the carb with the primer bulb.

Re: 70's 650 mercury problem

Thanks a lot for the advice will keep you posted after i get this done and take it for a spin.