Slowing down on top end.

[rx1fastervmax4]

At the last grass drag 400' good bottom and mid but after that it slows down. It likes peeks out and flattens out. Running 9200 rpm and engagement 4300 rpm and I do run bender box. Thinking it might be in secondary clutch but not sure.

 

[The-Hoss!]

I have the same problem. Took the secondary apart inspected the helix, it had considerable amounts of belt dust caked on it.
I reckon that is my culprit...