How do you adjust the idle screw on a 1999 Honda TRX 250 Recon?

The screw on the upper side of the carb is the idle speed adjustment. The air to fuel or air screw is located lower on the bottom of the carb, right in front of the carb bowl.

Procedure: First, warm the engine by driving the ATV for ten minutes or more, or let it idle. It has to be done outside in the open air, doing this in a closed garage can lead to sudden death from carbon monoxide poisoning. First, locate the idle adjustment screw. With the engine running, give this a few turns until you know how to increase and decrease the engine speed. Now, listen to the engine and twist the idle adjustment screw slowly from right to left or counter-clockwise. The engine speed will begin to drop. Now go on to turn the screw counter-clockwise until you hear the engine begin to stall or miss. When this happens, you turn the screw back half a in the other direction or clockwise.

For the air screw, to get a richer mixture then tighten this screw. To lean it out, loosen it. Do this in small steps, then try revvying and driving the ATV between adjustments until you are satisfied with the way it runs. A very lean air mixture will make the ATV backfire on deceleration. If this happens, enrich the mixture a bit.