We tuned a 2017 KTM 300 XC-W with the Mikuni TMX38 carb for a few weeks by trying different jets and needles, adjusting the float height etc. No matter what we did, it just didn’t run well consistently. It eventually started dumping fuel on the ground while sitting in the garage a few minutes after riding.
We found that the float was absorbing fuel.
The float would absorb fuel and get heavier as the bike was ridden. When the bike sat for a few days and the fuel level dropped in the bowl, the float would dry up and get lighter. This was preventing any chance of having a consistent fuel level in the bowl. We ordered a new float, set the height with our Mikuni TMX 38 Float Height Tool, and all was good.
If you’re having similar issues, you can check your float by doing this: take the float off the carb and let it dry out overnight. The next day, fill up a small container with fuel and put the float in the fuel. The float should float level in the fuel. Let it sit for a few hours and come back to check on it. If it’s sitting lower in the fuel, its absorbing fuel. The float in our bike only had one side absorbing fuel, so it was floating at an angle.