i just put a new piston in my xt 350. starting it up with the choke on the bike runs. sounds fine 2000-3000rpm with the choke on, or the choke at 1/2 on. when i put the choke all the way off it sputters and dies. even if i hold the throttle at 3000 rpm there will be the sputter it might not die right away though. but if i let it go to idle no choke below 2000rpm. dies pretty much instantly. when it sputters there is air coming back through the carbs, as in through the air filter.