Backfiring on start-up

So here is the latest problem, when I go to start my 05" WR450 she backfires and is kinda hard to start. Once she starts, runs great, no stumbling, not popping on deceleration, and she screams. I have about 350 miles total on the bike, done all of the free mods, JD kit, Zip-Ty, AIS removed, open air box, & open pipe. Thoughts anyone . ( valve adj?) :applause:

72 starter jet, 48 pilot jet and an adjustable fuel screw! :applause:

Thanks Indy, I have the 48 pilot, 168 main, and the fuel screw out 1.5-1.75, and JD blue #3. This started happening after I put a YZ pipe on. Could I be running lean? 4000', 50-70 deg. I've seen your recommendations and are following them but with the new pipe it's got me thinking that I'm too lean. Although I do have a 1/4 throttle stumble now which I believe means that I'M running fat :applause: Hell I don't know. Any help would be greatly appreciated.

Put in the 72 starter jet it will make cold starting much easier.

Put in the JD red needle on clip #5 from the top. It will run better at 1/4 throttle if the blue needle is too rich. :applause:

Thanx............ :applause::ride::p

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