My moped will not accelerate?

I just bought a 50cc 2007 roketa maui-50 and when I go to turn the throttle it dies out I can not even hit 10 mph but just yesterday when I got it I was going just fine hitting a top speed of 50mph can anyone tell me what I may need to do? I have already cleaned the carb changed the oil and cleaned the spark plug so what could it be?