Bad, dirty or fouled plugs could be the cause of a rpm loss, the only way to tell for sure is to change them and run it and see what happens.. I have personally only run the NGK plugs in my yamahas as that is what they came with and also what Hydro-tec installs.. From what I have heard, the champion plugs are made to have the electrode fail before burning a hole in the piston, which would work well in a nitrous or supercharged, or turbo application (the theroy we followed when building drag car or really hot street car engines).. Now after that side bar, back to your original deal, if you are doing alot of idling around, and low rpm running with your yamaha, maybe consider going to a 1 step hotter plug, a #5 NGK, as this will help to burn off the extra oil at an idle.. A hotter plug will put the electrode further into the combustion chamber causing it to be a little closer to the flame front and in the heat of combustion. Hope this helps, sorry it seems that I rambled on.. Good luck..