Take it apart, clean it up with carb cleaner, take 300 grit sand paper and polish the carbon off of the valve, coat the moving peices with a little 2 stroke oil, and put it back the same way you took it apart.
-I think on those models you need to be careful about changing the valve timing, but that may only be when you take the cylinder off.
There are gears under the side cover that controls the valve timing. Just ignore that cover and take the cover on the front off. If you need, take some pics as you take it apart, to help you get it back together properly. Look at a parts fiche for your bike, and you should be able to figure out how everything works.
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