new to scooters - need help

[McNutts]

Howdy. I just bought a taotao evo 150. it's used with about 1200km on it. it runs and the previous owner turned up the throttle screw since he was a big guy. I rode it around for a test run. it wouldn't go above 40mph and the RPMs never went above 5000,that was right at take-up then quickly settled for 4500 RPM. I adjusted the throttle screw back down but the only thing that changed was it wouldn't try to drive itself at idle. I don't know where to begin to try to figure out why it is so slow/low RPMs. Any advice would be super. you might need to dumb it down for me as I don't know squat about the proper names for stuff.