Below is a republished article of Dave Closs and Brian Jackson's XR650L carb mods that was posted in our Forums. These mods may correct the stock lean condition thus allowing the motor to run much cooler. Also the low and mid range power will increase substantially and throttle response will be liquid smooth compared to stock. Brian considers this almost mandatory for any XR650L. Though intended for 93-04 XR650Ls, these mods may work on other model bikes that use the Keihin 42.5mm diaphragm-type CV carb.

1. Be clean as possible. Clean the carb exterior really well before removing it so nothing gets into the intake manifold or head. Always clean out all metal shavings and dust from the carb before reassembly. Before starting any work run your carburetor dry of gas. Do this by running the engine with the fuel petcock valve off until it dies (runs out of fuel).
2. Make sure the fuel petcock valve is set to off. Disconnect the fuel line at the carb and remove the seat and fuel tank.
3. Remove the throttle cables, hoses, choke cable, fuel line, rear brake reservoir and its bracket. Make sure to note all connections so you can put things back to where they were. (Figure 1 & 2)
4. Loosen the hose clamps on either end of the carb.
5. Squeeze the intake tube (from the airbox to the carb) back and out of the way. You can pull the carb out of the frame now to the right side quite easily. Cover the intake to the head with at least a clean towel. Make sure not even dust gets in there.
6. Remove the top and bottom plates of the carb by removing 4 screws on each end. (Figure 3)
7. Remove mixture screw now that it can be turned. (Figure 4)
8. Grind off the tab on the mixture screw as shown in figure 5 and 6, or as Dave did, grind the tab off the carb body instead of the mixture screw. See figure 7 for the tab on the carb.
9. Reinstall by turning all the way to the bottom, lightly seating it and then backing it out appropriately. (Most seem to think 2-1/4 to 2-1/2 will run best.) (Figure 7)
10. Remove large spring and slide from top of carb, under the top cover.
11. The Needle: When removing the slide, the needle is held into the bottom of the slide by a little yellow widget that you press down on and turn a little with a screwdriver or an 8mm socket. When you remove the little yellow widget, the needle comes right out. Notice the needle sits in a small recess. Find a small washer that is 0.020-0.030" thick and fits fairly snugly over the needle at the thickest part at the top. The needle will be raised by the thickness of the washer. If you don't already have a washer that works, check your local hardware store. Some folks have found a similar washer in a kit from Radio Shack.
12. In the bottom of the slide, drill out the two existing holes to 5/32” per the photo. (Figure 9)
13. Reinstall with washer, as removed earlier. Make sure to seat the slide bowl (terminology?) properly and align the tab correctly. (Figure 3)
14. On the bottom of the carb, remove the plastic piece (slosh baffle) that is partially covering the pilot/slow jet. (Figure 10) Note: If you use the longer style pilot/slow jet, you will need to clearance the slosh baffle to accommodate it.
15. Gently unscrew the brass jets. The longer larger one is the main jet and the shorter flush-mount one on the pilot or slow jet. Gently screw in the new jets. (Figure 11)
16. Place the plastic piece back over the main jet and install the carburetor bowl. (Figure 10)
17. That’s the end of the carb mods. Check for smooth operation of everything, clean well with carb cleaner, and reinstall the carb on the bike. After installation, recheck the operation of the throttle and adjust of necessary.