I thought I would ask if the engine was used when you purchased it?
Do you know if it was disassembled and reassembled before too?
someone might have put in a reverse rotation camshaft previously.

But I would start with checking the valve gaps first in case they gaps are way off.
If that didn't seem to be the problem...
Then you want to check the camshaft timing to ensure it isn't off a tooth or more.

What fuel are you using? How much nitromethane are you using. Some guys can have problems running 20% or more fuel in the engines. 5% to 15% nitro glow fuel is more than adequate for most needs.