Sled works fine in reverse but doesn't work in forward

[AkRider217]

I recently took apart the whole top end of my 2003 RMK 800 because coolant had got into it. I cleaned out everything including my exhaust valves and replaced all the gaskets and O-Rings. I bled the coolant lines too. Now when I get it up to 2000 RPM and above it starts backfiring like crazy. But when i put it into reverse everything works fine. I've never heard of anybody else having this issue and have no idea what's going on.

 

[AkRider217]

Is there anybody that can help with this it's very disappointing to be out for the season in spring in Alaska.