What kind of head gasket are you using? If it is hi-temp form-a-gasket silocone stuff it could be preventing heat transfer between the cylinder and the cylinder head. Because these engines are air-cooled, spreading the heat over a larger area helps them stay cooler.
Also, what kind of gas r u using? I have heard that higher octane gas burns a bit hotter, but maybe someone else knows better than I do.
You may also be running lean, meaning you have too much air in your gas/air mix. This can be corrected by up-jetting or restricting air-flow.