It's best to keep the engine clean on the outside because any varnish acts as an insulator so the engine will run hotter. I haven't tried it myself but apparently carb cleaner is very good and doesn't involve any dismantling or poisons. When the outside is clean, fit a BruLine air filter which not only helps keep dirt out but also stops fuel from spitting back out of the carb. This is the major cause of varnish in the first place.