I have two of these engines and really like them.  There is a lot of information here - RCU - on guys that have had similar problems as you are having.  I was one of them early on.  Bill Baxter from Hobby Services/Tower/OS knows a lot about them.  Look up his responses on here and you will learn a lot.   Anyway... here's what worked for me.

First the 13-8 is perfect for this engine.  The only downside on my Skybolt is that it doesn't want to slow down for landing in calm air.  I need a breeze to help.  The props quoted in the manual are too big for good performance.  In Japan noise is an issue, and the larger props help with that.  The 91 likes to rev, but 13,000+ is too high.  Detonation is probably occuring at that lean setting.  The noise you hear is the engine going lean in the air when it gets hot (too hot).  The fuel feed is part of the issue as well.  My solution was a $30 Perry pump.  It provided incredible power immediately, and didn't require anything except reading the instructions the first time.  The noises went away, the plane turned into a vertical rocket (with the 13-8), is fuel efficient,  and with a slimline pitts on it makes a very cool Harley kind of sound at idle.  Jeez it's great.  I use an OS#8 and 20% heli fuel.  Smoke trail is awesome and it is there through the whole flight.  If I get off a click lean, the smoke thins out and that's my clue.  Anyway, 13-8, Perry, 15-20% fuel.... and hang on.    -Tony Iannucelli