Not keen on the 'add to fuel' solutions, for a variety of reasons.
One product I used extensively when I had a dealership workshop was Merc's Power Tune. Run the engine, at idle on flushers until warm, then increase the throttle while spraying this stuff straight into the carbs. Then, while still spraying, let the engine stall out. Let the power tune sit for 20 to 30 minutes, then fire up the engine. Smoky as billy-o! And watch the run off water. It'll be black with carbon. If you want to de-carbon a piston/combustion chamber or have a few gummed up rings, this stuff is the real deal, but if the problem is a worn engine, or any other damage, nothings coming out of a can will fix that...
Chris......