I use a crockpot of ethylene glycol antifreeze. Pull the muffler, glowplug, backplate, and remove any plastic parts and drop it in overnight. Baked-on varnish comes off like magic. The next morning, I flush it thoroughly under hot water from the faucett and wash it with dish soap, turning the crank over several times. I then rinse again under hot water, turning the crank now and then to get all soap out. Then I put it in the oven at the lowest setting (mine is 175F) for 30 minutes or so to bake out the water (take any plastic parts off before baking). Then I let it cool, liberally oil the innards with ATF, and reassemble. Others may be scared of having their bearings immersed in water, but if you go quickly from the final flush to the oven you will have no problem. I have cleaned dozens of engines this way, after finding no other effective ways (solvents, oven-cleaner, alcohol soak, degreasers, etc) to cut the crud from the outside. WARNING!!! WARNING!!! WARNING!!!: 1) CRITTERS LOVE THE TASTE OF ANTIFREZE AND IT IS DEADLY TO THEM. IF YOU LOVE YOUR CATS AND DOGS KEEP THE CROCKPOT AWAY FROM THEM. 2) DON'T USE THE CROCKPOT AGAIN FOR FOOD! For smaller engines I use a small coffeemaker so I don't have to use as much antifreeze.