My Yamaha yz250F 07 starts first kick but back

I see: you changed the plug because it was backfiring.

Does it back fire when you let OFF the throttle after revving it, or does it back fire while being revved?

If it does it when you let off then Ollie's suggeston might do it. If that doesn't do it then check for dirt in your pilot jet.

If it back fires when revved up high could be dirt in your main jet.

Ok buddy give this a wiz on the carb will be the air mixture screw fiddle with this screw and if nothing happens set it back to where it was record how many turns you turn it i i reccomend