You could also check to see if any of the bolts holding the cylinder or head on are loose....if they are, that could be causing your air leak. If you've run it for a while like that (and were lucky enough not to blow it up) then you should probably pull it off and put a new set of gaskets on. If you had just rebuilt it and started it up and putted around and let it cool and then found that it was loose, tightening it up should suffice. It's hard to tell when new gaskets will be needed and when not so it's best to err on the side of caution, IMO.