After fighting this engine for a whole day somebody at the airfield brought up some heli "powermaster" fuel 30% nitro. After having very dismal performance out of the 15% Byron Nitro and what seemed like every prop at the field, we thought,"sure...why not...can't work any worse." So we fueled it up and cranked it over...Couldn't believe the difference. Most importantly the Needle Valve had a much broader range and all the air bubbles went away. Instead of four-stroke rich, two clicks, dangerously lean you could actually adjust it. We ran a tank through it, then filled it up again and flew the model through two more tanks. Upon further inspection the engine was much cooler than ever before. We switched back to the Byron 15% for the next tank and it is still running much better. Nobody could exactly explain the phenomena, but it worked. The best explanation the most experienced guy had (professional model airplane person), was that the piston was in way too much contact with cylinder wall to function properly and while trying to break itself in it was generating way too much heat causing a chain reaction of other issues and potential damage. His advice was to break these engines in with very-cool, high nitro fuel to get the ring broke in and then you should be ok. Sounds good, and I certainly hope he is on the money...(because its my money in the air ;-) ). I am still a little trepedatios about this engine what with my past experience with OS 91 FX #1 that got sent back and then seeing my friend's 91 work well for three months and then go kaput in one flight exhibiting all the characteristics that were mentioned in all the other threads (he has placed a super tiger 70 carb on it and it seems to be working much better but he has noticed the decline in power since the modification). Anyway, I hope this may helps some of you disappointed 91 fx owners, but I will still get and, keep a K&B needle valve at the ready.