I'm new also and just got 4-Stroke Pull Start 48cc Bicycle Engine Kit EPA Certified BikeBerry.com
about 2 months ago. I wanted a 4 stroke since I plan on using it as my main ride(recently developed epilepsy and don't wanna kill someone else) and thought it would be nice to be able to fill up at the gas station. The kit didn't come with instructions but after emailing them they sent me a PDF for one just like it.
It runs very well and will push my 26" mountain bike at about 30 MPH all day long(I weigh 250 Lbs) and slows down to about 20 going up steep hills. Gets about 120 mpg with lots of stopping at 4 way stops in town. It has a centripetal clutch so you don't have to kill the motor when you stop/hold down a clutch. Runs kinda like an automatic transmission that's always in drive. Only problem I've had with it is when the tank is full it sometimes will put a vacuum on the fuel tank and bog down/kill the motor. Popping off the fuel cap fixes it right away. I planned on pop riveting a chain to the fuel cap anyway to prevent losing it or dropping it in dirt so that should fix the problem.
All in all I'd rate it 9.5 out of 10 because of the fuel tank vacuum problem but everything else about the kit is great. Very happy with my decision to pay the extra money for a 4 stroke with a centripetal clutch. But then again I ride the heck out of mine. Hope that helps.
I did forget to mention I did a few mods to the kit. I got rid of the phillips head screws holding the muffler on and got some allen head screws from the local true value for about $1.50 because the muffler blocks you from getting in to tighten the screws well enough with a screwdriver and it kept loosening and blowing out the gasket, I also bought some high tack at a car parts store to help the muffler seal better. I cut off the factory end for the kill switch wire connectors and used some heat shrink butt connectors to keep moisture out better.