ok, i recently picked up an 01' yz426f. Its pretty much stock(except for the suspension and some other things on the chasis) and back fires when you let off the throttle. I cleaned the carb, moved the clip on the needle down 2 positions(its a kiehn FLAT CR carb), seems to rev a bit stronger after i did that, but still backfires when its warmed up. Before i did anything, i checked the plug, and it was black with white speckles/spots(detonation obviously) I also put fresh 93 octane in the bike. There is no exaust leak, and the intake manifold and rubber boot that connects the airbox to the carb are tight. should i move the clip down one more slot? The bike also sat for 2 years(which shouldn't factor in, because i just cleaned the carb, and put new gas in it) Any ideas?

 

Youre right to check for intake and exhaust leaks...the popping is caused by a lean idle circuit. So moving the needle position wont help or hurt a thing.
Is the exhaust stock? If it has been modified, and is really free flowing, then you might just have to live with a bit of popping.
Try going richer on the mixture screw, and if there isnt enough adjustment( if you get to a total of 4 turns out from lightly seated), go to a bigger pilot jet.