RAVE valves!
I agree with Joe, sounds like your running to rich. Especially if your noticing oil and carbon build up on your rave valves. There should be some light carbon build up over years of use, but no real oily residual. With the presence of oil, running rich, the motor's not getting hot enough to burn off the lubricating part of your oil. Careful on leaning it out though, to much can lead to serious damage.........and check your spark plugs, this will tell you if you need to lean out or enrichen.
Looking at our manuals for your engine, there is a paragraph I read in there about re-jetting the carbs for altitude and temperature. There's several pages on how to adjust your carbs from a dead start. Way to much detail for me to try and give you here, but detail that's easily understood by someone who has basic mechanic skills. A very important section on adjusting the oil injection pump if your carb adjustment changes these settings (provided your not using pre-mix instead).