hi i own a 1969 20hp johnson when tested in water tank every thing seemed ok , but when i took the boat out to make sure the engine was working fine it started ok, goes into gear ok but when on full throttle only went full speed for about 5 mins then seemed to lose power would only go half speed,checked sparkplugs sparking fine checked oil petrol mix 50to1 ok.engine seems to jump as if it wants to go full speed. engine has not been used for 18 months. thanks david
