If it makes a "whaaaaa" sound when you open the throttle, it's probably running lean.
If it runs "lumpy" when you open the throttle, it's running too rich.
Lean is usually caused by crap in the carb, a wrongly assembled carb, or an air leak somewhere in the crankcase. But it could be other things too - fuel flow restricted etc.
Do you have a Walbro carb? if so check the order of the gaskets is correct. There are some useful links here: http://www.paramotor.co.nz/viewforum.ph ... 61e9db9d41
To test for an air leak, you have to block off the intake and exhaust ports (with plugs or plates and gaskets), then pressurize the crankcase with around 6psi, and see if this holds over a period of ~5 minutes.
If it doesn't hold pressure, get soapy water and spread this around the entire engine - bubbles will form where the air leak is:
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