Well either the pipe is not suited to forest work or the jetting needs dialling. The pipe will affect power in relation to revs. Think of the jets in relation to throttle position rather than revs.
Try turning the idle screw in for richer & out for leaner & check if one way feels better. If it does try a pilot jet in that direction. Also try moving the needle clip in each direction which may give you an idea.
Keep a watch on throttle position. If the problem occurs when full open then main should be played with.
If no jet tinkering helps, & it is throttle independent then time to rethink the pipe.
Good luck.