Recently bought an 08 wr450 with low hours. I bought it jetted for sea level with a fmf Q4 pipe, snorkel removed, and grey wire cut. All of which I was told was done by the dealer when the guy bought it new. I believe him. it ripped and pulled the wheel up easy in 3rd. I live at ~4500 ft and do most of my riding between that and 6000. Still rode fine when I got it home just noticed a little less power and throttle response. I put about 400 miles on it with no issues before my jet kits came in the mail. got the GYTR AIS removal kit, JD jet kit, RD flex jet, and a boyesen quickshot. Did the AIS removal, put in the red needle from the JD kit (clip 3rd from the top), Swapped a 175 main jet for a 168, swapped a 50 pilot for a 48. Put the thick O-ring on the AP timing screw. Installed the flex jet and set to 2 turns out. Set the Boyesen to ~ 45-50. I rode it to work the following morning (~30 miles. most of which done in low to mid 5th) and it all seamed great. Felt like it had more power but still needed some tuning (still had somewhat of a off idle bog if I ripped on the throttle). All in all I felt pretty good about it. Then on my way home from work about half way back it started sputtering pretty bad with some occasional popping coming off the throttle and a significant decrease in power. I figured I must of left something in the carb with my dirt hands. It felt very similar to other motorcycles prior to a good carb clean. I pulled the thing out again that night and went through it all with some carb cleaner and some air. Rode it to work again, felt great. But on the way home the same thing happened, but this time started earlier in the ride and got worse than before. I've got a fair amount of experience working on these kinds of things. Never had any problems with carbs or jetting that I couldn't fix or at least determine a problem. Never really truly fine tuned a carb, I just throw the approximate right jets in there and if its better than before, idles nice and accelerates good I usually leave it at that. I've been reading forums nonstop since I got the bike. Don't think I've gone to bed before 1 am since. But with all that researching I have not come across a similar issue. Granted I have specifically been looking for performance mods and jetting, and not for solutions to issues because, I didn't have any issues. What could possibly be the issue? Why would if run great in the morning for an hour and then fail so bad on the way back? In the morning its pretty cool out. I have to layer up. I work in the NV Desert so the bike sits outside all day in the sun. When I ride back its pretty warm out. 80-90. That made me think I was over heating or low on coolant. But the radiators are full, and I don't get that high pressure coolant overflow. The last 5 miles or so of my ride in and beginning of my ride back is dirt road which I rip on, spinning the rear wheel pretty good, but no harder than my average trail ride. Before I re jetted I had the thing in the sand for a while doing some hill climbs without issue. I'd like to get some opinions before I pull the carb back off again and either try one more clean, or put the old jets back in. Please Help.