I just bought a boat run by an old Homelite 55 hp outboard motor. it starts right up and runs fine from idle speed to about half throttle, but when i push the throttle down any further, the motor makes a slightly different sound but doesn't gain any speed, and after a few seconds it will die. what could be the cause of this? i suspect it to be gummed up carburetors, as the motor has been sitting in a barn for the past year. i've been running a fuel additive through the system to help clean them out but haven't had any luck yet. any suggestions?