There are several other things to check.
New engines will not develop full horsepower until they have been broken-in. Depending on the engine and machining tollarances between the piston, cylinder & rings, this could take from 5 ~ 30 hrs. of operation.
1. What height of cut is your deck set at?
2. There may be a bad bearing on a deck spindle or idler. Check
to see that the blades rotate freely.
3. Make sure that the mower belt is routed correctly and not
rubbing on anything.
4. Have you tried switching from one tank to the other? Maybe
there is debris in one of the fuel lines.
5. Do you see black smoke when the unit is operating under
load? If so, check to see if the choke is adjusted correctly and
that it is not restricting air flow.
Mission: Constant Never-ending Improvement