I'm working on a Coolster Challenger 250cc kart trying to get the top speed it had before back. Used to do almost 50mph and now it maxes out at about 29? I was here last year with this problem and thought it was the clutch but it's been replaced and is working but the engine has no high rpm. I have the rear jacked up and I can hold full throttle and it just hits a rpm wall at a little over half throttle and that's it. Plug was black so I thought maybe it was too rich so I swapped the carb with a Keihin carb that I think was original and it didn't help. Adjusted the valves too with no change. Idles and throttle response is good but refuses to rev high. Any ideas? TIA