Hi guys! I bought a 125cc 3 speed with reverse from China, installed it on my homemade go kart (used to have a 6.5hp). I test ride the first time yesterday and it was disappointing. I'm not sure whether the engine is defective or not, shifting from 1st to 2nd makes almost no difference at all, the engine barely accelerate and it just revs very high, forcing me to shift into 3rd gear. I thought the gearing is too low, 14T to 41T (the tires are 13”). Then I change it to 17T to 37T. It is better but still the go kart is noticeably slower than before. The engine just vibrates and revs too high. I thought the 3 speed would be an upgrade and gain more speed Does anyone has experience with these 125cc engine before? Please give me some opinions. Should I swap the old engine back. Is it a bad idea after all?