Homelite 55 hp motor not running up to speed

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?

540 Views & not 1 reply - sounds about right ...

Is that In a 44 Gallon drum or on a boat under load.... ???

Some would pay a lot to be told nothing (basically) given : " Yikes NO ECU - Yikes NO CDI , YIKES it was 30 years ahead of its time - but don't ask me - I'll sell you a nice New 50hp Honda ! "

Check for collapsing Fuel line bulb/fuel-line .... Weak Fuel pump (clean the gauze in the sediment bowl ) - check the Tell-tail is still Squirting water when the Eng note changes ... whats the eng serial number ?
Is it a GRAND PRIX with The Shroud Covering the Carb inlets ???? (If its in a Drum/Tank when it sags , theses probably half your answer)

(no wait for the experts to no front)

Does the Powerhead/Exh Manifold become too hot to comfortably touch AFTER it sags right after shut-down ??

You won't find the 2-3 Bearcat (US) repair/rebuild Firms replying here (the ones that bought up most of NOS ex Feebay circa 10 yrs back) as they'll be guarding learned info via experience & their Income $$$ (don't blame them given somes M.O.'s I've sadly seen)

Give me the Serial number & check the obvious & get back & I may be able to help you further
Cheers