What you *can* adjust are things that *indirectly* affect the air/fuel mixture:
* Fuel Screw (Pilot Circuit): While it's not a direct air/fuel mixture screw in the same way as a typical slide carburetor, the fuel screw on the TMX does affect the pilot circuit, which is responsible for the mixture at idle and low throttle openings. This is adjusted with the engine running. Turning it in (clockwise) leans the mixture, and turning it out (counter-clockwise) enriches it. However, it's generally not recommended to adjust this unless you have a very specific reason and understand its effect. Incorrect adjustment can lead to poor running or engine damage.
* Needle Position: The carburetor's needle controls the fuel delivery through the mid-range. The needle jet is clipped onto the needle, and moving the clip up or down on the needle changes the fuel delivery. Moving the clip up leans the mixture, and moving it down enriches it. This is a more significant adjustment than the fuel screw. Again, this requires a good understanding of jetting and should only be done after careful consideration and possibly with the help of a professional.
* Main Jet: This jet controls the fuel flow at wide-open throttle (WOT). Changing the main jet size is another way to alter the air/fuel mixture, but this is a more drastic change and should only be done if you've ruled out other issues.
Before making *any* adjustments:
* Ensure your bike is properly maintained: A dirty air filter, clogged carburetor, or other maintenance issues will affect the air/fuel mixture. Address these first.
* Identify the problem: Why do you think you need to adjust the air/fuel mixture? Is the bike running too lean (overheating, pinging, poor throttle response)? Or too rich (fouled spark plug, poor performance, smoky exhaust)? Diagnosing the problem correctly is crucial before making any adjustments.
In summary: You can't directly adjust the air/fuel mixture on a 2003 YZ125's carburetor in the same way you would on older bikes with a readily accessible mixture screw. Changes to fuel screw, needle position, or main jet are indirect methods and should only be done by someone with experience and a good understanding of carburetor tuning. If you're not experienced, it's best to take your bike to a qualified motorcycle mechanic. Improper adjustments can damage your engine.
