TAO 3 speed clutch slipping 107 cc

Hello everyone, I have a 2013 TAO 107 cc chain driven Chinese go kart with a 3 speed manual transmission. I have changed the engine oil, cleaned the carburetor, but the clutch is slipping at about half throttle or more. RPMs increase, but it slows down at about half throttle or more. Cannot seem to find any information on this three speed model. Does it take a separate oil from the engine oil in the transmission? Wondering if the transmission oil could be low or old? Thank you in advance.