i've had some bad ones from aquired engines, one so bad the valves and piston (side valve) was hitting the head, producing a pinging sound, not to mention it was a bitch to start. if its light, a 3m scotch bright pad, and maybe a paint scraper for the piston will do when its heavy, then go at it with the pad. no real need to oil anything but the bore after wiping it clean as well from any carb that has settled on it. and on engines that have been setting out for years, unused and left for dead, that carbon mostly pops and flakes off, so when you go to get it back to life, pop the head and clean em. but there is not real need to get them spotless, or every last bit of it off, but they should be cleaned atleast every 3 years or so if used for homeowner use, it may not always be that spark plug that needs to be replaced if it gets hard to start, but a combustion chamber cleaning, clean em even more if they use some oil.