The procedure to decarbonize (sometimes called decarburize) engine and pipe is as follows.
Stainless steel (the crankshaft and liner set are of stainless steel) is easy: just put them in a chlorine solution (used for cleaning the toilet) for an hour and all carbon just falls off.
Aluminium (most of the engine) is less easy. Do NOT put aluminium in chlorine as aluminium will dissolve!! One safe but time-taking way is to burn away the carbon. I used a pencil torch heater, and burned away the carbon millimeter by millimeter. The fine but very hot flame of the pencil torch heats up the engine very locally, avoiding overheating or melting, while at the same time being hot enough to burn the coal. Another method, better suited for other aluminium engine parts and pipe is to soak them in washing powder dissolved in COLD water for half an hour (biotex is a good brand for this). Do not use warm water, I heard that the aluminium will turn black! After half an hour you can then clean the stuff easily with a toothbrush, or with your finger if the surface can easily be reached.
Another method is to cook the muffler a few minutes in dishwasher detergent, cook it in antifreeze (unknown to me how long or how hot), or use oven cleaner. They say the oven cleaner is very agressive, so be careful and dont inhale the fumes. I never tried these methods so I can't comment on them.
A pencil torch running on cigarette-lighter fuel (buthane). Sorry for the barrel distortion :-)