I recently purchased a used 07 WR450F from a dealership. From what I can tell, the original owner did all the free mods and used a GTYR kit. It came with a 2Bros M7 system which was to loud for me, so I got a FMF Powerbomb Q4 system. I also added a JD Fuel adjustment screw. I rode it around town for awhile setting the idle and fuel screw. It ran great, plenty of power, way quieter than the 2Bros M7, but a nice growl when you get after it. Time for a long ride test. I head for the country, with a lot of higher speed cruising, got the bike good and warmed up. Around the 30-35 miles, I start noticing backfiring on decel, and slowing down to turn, it almost wants to die. I rode the bike with it performing that way until 60 miles, when I switch to reserve, and head for my farm to re-fuel. I get to the farm, fill it up with gas and head back for town. Now the bike is running great again, NO backfiring on decel, and doesn't want to die when slowing down. Thinking back I had the same issue with the 2Bros system, as the first tank of fuel got low, it started backfiring on decel and ran like crap. I thought bad fuel or gunk build-up, so I took the carburetor apart for cleaning and a "get-to-know" my carb session. When the bike ran fine after cleaning and installing the Q4 system, I thought I had solved the problem. I didn't link the backfire/poor performance to an emptying gas tank until it happened the 2nd time, and is working great again on a full tank. Anybody ever had this issue or have any ideas as to what I should be looking for? Specs are in my signature.