Gy6 clone runs fine replaced carb so not stalling but wont go over 30 MPH!

I recently bought a used 150CC GY6 clone moped and when i got it it was dying out. Sputtering at full throttle. Went to a shop and they replaced the carb. Moped ran fine but still wouldnt go over 42MPH. I could deal with that but after having it 3 days i noticed rpm guage was bouncing all over dail then it slowed down to to a top speed of 29MPH so i took spark plug out and clesned it. Now my rpm guage is maxing out about 6k and staying there but speed is staying at 29MPH at full throttle, but no sputtering and motor sounds fime MAYBE not full throttle loud but it runs fin slowly. Please help thiscis my only transportation to feed my family!

 

Hard to say really sounds like you have more than one problem. Very difficult to tell without seeing it. The rollers could be worn from stopping you get to a higher speed.
Have you checked the carburettor is opening fully with the throttle ?

 

Could be the throttle cable actually