I am trying to sort out this 2003 WR450F I just bought so that it runs well. When I bought it, the carb was completely gummed up from having sat in a garage for more than 3 years. I took it to the Yamaha dealer and he says he rebuilt the carb and jetted it appropriately for the high-altitude riding we do here. Well, I got the bike home and it starts easily -- first kick -- as long as the choke is used (hot or cold), but when I ride the bike, it pops and backfires on deceleration. That suggests a pilot circuit that is too lean to me. It's kinda obnoxious as the FMF Titanium exhaust (including a "Power Bomb" header) on the bike is quite loud (despite having added FMF's "Quiet Core" insert and their Spark Arrestor/End Cap). I live in the mountains at 8200 feet above SL and if anything, the bike should be on the rich side -- because the air is thinner here. At least that's how it seems to me. Do I take the bike back to the dealer and ask that he put a bigger pilot jet in it -- or is this backfiring just the way these things run? TIA for your input.