I would not suggest WD40 for your engine. One of the best ways to clean the engine is to remove the plug and immerse the entire engine in fuel. If you are going to disassemble the engine just remove the back plate and plug before you dunk it. Drain thoroughly. If you have a compressor blow dry with air at low pressure. Use a good after-run oil on all parts. If you plan to store the engine for an extended period be sure you have oiled all parts thoroughly, and periodically,every two or three months, turn it over with the plug removed. If there is the least bit of resistance to free turning go back to step one and repeat the process. If your engine turns dark on the exterior due to burned on varnish check with your hobby shop for an exterior cleaner. If the outside coating is very heavy your engine is running too hot. If this is due to a cowling open the air outlet so that it is at least three times the size of the inlet opening.