There's a special tool sold to do it, but you can do it carefully with a small and medium screw driver. If you look on the driver's side of the carburetor, there's a ring adjuster with two slots that locks down on a slotted adjuster. Find a flat screwdriver that fits the lock ring on both slots and carefully break it loose just a tiny bit....but not loose enough to unload the inner slotted adjuster. That adjuster isn't threaded and if you loosen the lock ring a lot quickly, the center adjuster will unload quick and sometimes makes the spring come off the inside. Then you'll have to remove the carburetor top to fix it. Not the end of the world, but being careful will keep that from happening.
So just barely loosen the lock ring just a little. Now, turn the inside adjuster counter clockwise with the smaller screwdriver to tighten the air door. You can take your finger and feel the tension on the door as it tightens up.
