Hi guys! I'm new to this forum, hoping to get some help here. So I bought an ATV engine from china, its a 125cc 3 gears with reverse. I'm not really sure what model is that engine but i think its the same engine that is found on most chinese quads. I plan to replace the original 6.5hp engine with it on my homemade go kart. So I finally got it to work yesterday after some work. Its my first time messing with these type of engine and not sure what have I done wrongly. The first gear seems fine but the second gear is literally not doing anything here. I mean after I shift into second gear, the engine doesn't accelerate much and it just revs so high and vibrate a lot (sounds like crap too) and I'll have to shift into 3rd gear, the 3rd gear is fine i guess but the kart is noticeably slower now. The main problem here is the engine revs very high but doesn't move much? Or its just the way how these engines are built? I know they are chinese made, but I was expecting a better performance since it has 3 gears compare to my old engine. I have already tried to adjust the clutch and mess with the carb, what else can I possibly try? Please share your opinion with me guys

 

Seems like you have got a dud if second gear doesn't work. You don't say what type of 125cc engine it is, but the GY6 and Minarellis are belt drive autos so, as it is manual, probably a Honda Cub clone, often known as the E22 Don't think Honda made a 125cc version but the old 110cc Honda is similar and, if you can't send the engine back, use a Honda Workshop manual to fix the fault.

You will have to change gear ratios on the kart as the geared down output sprocket on the new motor probably won't be going as fast as the output shaft from your old engine.