My OS 40LA seized up this weekend while I was starting it. It felt like it had ingested a piece of wood because I could still prop it over by hand but it was tight tight tight at the top. Today I took it apart expecting to find something in the cylinder and found it to be clear of debris but it was gummed up. The top of the piston is sooty where as the sides , especially near the top, are coated with a black varnishy residue. Now, I know this has something to do with fuels and lean/rich settings....but I have been flying the same type 15% Castor based fuel for about 2 years and the same needle set for 4 previous flights that day. What do you suppose got into this engine?
What is a good way to clean the residue from the parts.... I surely don't want to polish the piston or liner clean because this would change the tolerances. By the way, the crankcase area...rod crank etc is spotless.