the power valves (also called RC valves, or flapper valves, etc.) are in the exhaust port of the cylinder. And yes, in order to clean them properly, you should remove them and then scrape off all the carbon build up, then polish them clean. Generally the pockets the valves sit in don't get build up, but the exhaust port does, so you want to clean that area up really well.

then he exhaust pipes themselves can get a good amount of build up. There are special tools for cleaning expansion chambers. Basically like a unravled wire cable that attaches to a drill. stick it in, turn on the drill and clean away. Of course with the stock exhaust, you got baffles and stuff inside the exhaust so you may not want to go all the way in with the tool.
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Charles Gallant