I messed with a 66cc SkyHawk for about 1,100 miles. It still runs the same as the day I got it. It had pretty variable performance most of the time. Didn't want to run in the cold, ran rich and was sluggish in the heat... It didn't have tons of power, but it ran. It was dependable. I followed the break-in procedure down to the letter-- four tanks of gas at 20:1 for no more than 10 minutes at a time, with at least 20 minutes of cooldown between runs, at no faster than 15 mph. After all that, it did not perform any better than it did the day I got it. The performance had not changed in any way after 1,000 miles either. It still started quite easily and I never had running problems. It was reliable. It ran. But it had very little power, and getting through left turns in traffic was sometimes embarrassing because the motor just didn't want to go. (Just in case anyone responds by suggesting there might have been an intake leak, etc., I'd like you to know what I did to the motor: All (and I mean all) the gaskets were replaced, and "The Right Stuff" was used to seal them. New intake and exhaust gaskets were installed, the intake had teflon tape wrapped around the connection to the carb, along with an o-ring, fuel mix was the dealer-recommended 20:1 ratio, compression was good, spark was good, the carb was rebuilt once and replaced once, no intake or exhaust restrictions, electrical connections were soldered and wrapped with heat shrink, the fuel line (with new in-line fuel filter) was clean, nuts/bolts were torqued to specs, motor was consistently checked for vacuum leaks, and all nuts/bolts/screws were checked (the entire bike and engine) every 30 miles.... Many hours were spent overhauling, repairing, and maintaining the motor and bicycle, and never was a single problem found. I am also an ASE-certified mechanic. Thank you. I'll get off my soap box now.) A few days ago, I finished rebuilding my bike with a HuaSheng 4-stroke motor from Spooky Tooth. It tries to rip my rear wheel out from under me every time I hit the throttle. The torque is unbelievable. I weigh 210 lbs and am running a 44-tooth rear wheel sprocket with an 11-tooth drive gear, and I have no problems with hills. The exhaust is no louder than a china girl engine, and there is almost no engine noise whatsoever. I will never, ever, ever, EVER, EVEREVER go back to any china girl engine. The HuaSheng is $300 plus shipping and worth every penny. And you know what I had to do to this engine when I got it? Install it and ride it.