Sounds like the low end is lean. Lean the low end by turning the the low end in about a half till it gets rich. The low end on YS motors are oppisite then the high end.

Also check the regulator. Make sure it is flush with the housing.

You may want to break in the motor in the air. Or it will get too hot. Run the motor rich and fly around at half thr. for about 5 mins.

Make sure the fuel has 18%-23% oil. More the nitro the more oil you need.

Mike