150cc GY6 scooter lost top speed

[sc00ter]

Are you sure you have the throttle cable ran correctly, and the slide is opening fully when you twist the throttle wide open? I've seen throttle cables ran wrong before, usually not seated correctly. There is also a barrel adjuster to take out the slack under the throttle. The throttle should only have a tiny bit of slack at idle.

So make sure the slide is completely opening and there is no excess slack in the throttle. I'll keep thinking......