Worked on a Yamaha 125 twin once that had been losing power, getting weaker steadily, did tune-ups cleaned carbs replaced ignition parts nothing worked. Finally it just quit altogether, spark plugs drowned, raw gas in the crankcase. Turned out 5,000 miles of running outboard motor oil had plugged the baffles in the mufflers and choked it to death. Wouldn't have happened with proper two cycle detergent motor oil. At the time we were selling a quality (Homelite) two cycle oil for less that half the price of that outboard stuff, but the customer knew what he wanted and I couldn't change his mind.