I live at 7000 ft, and I am having a hard time getting my engine to pull me any faster than 25-28 mph. I have port matched my manifolds and my needle is maxed out to the leanest setting. I am running the stock muffler, and Opti-2. My spark plug is a gray color, (alittle on the light side, but acceptable). I ordered my 66cc silver flying horse kit from bike berry and the ports on the intake and exaust looked clean out of the box. I have changed spark plugs to a NGS H7BS, because the engine was running hotter than what I have read here. My engine runs strong up to about 18-20 mph and then the exaust sound changes dramatically to a splatty sound and then the last bit of speed take for ever to max out. If I go down a hill with WOT it will get up to some scary speed, and then is seems to run a little better on the flat afterwords for about a mile. I am planning on ordering an expansion chamber in the very near future, but I was wondering if anyone has any tips first. Also, the performance seams to be speed based and not throttle possition. I say this because it will run fine WOT untill it reaches a sertain speed (rpm) and then it changes. Also, it will ramp to a really nice high rpm without the clutch engaged, but with it engaged I can only get to this rpm going down hill. The compression on this engine is high (I can't turn the engine over with one foot while holding the rear tire in the air with one hand as I have read some people can), and it has only went through one tank and a half. I am only looking for more speed so that there will be less cars passing me and less chance of being hit.