KTM 640 LC4 Mikuni BST40 emulsion tubes
Left is new. Right is worn with under 20K miles (32,187 km) (claimed). The green circles represent the boundary of the standard bore diameter. On the worn example, the black area outside the boundary of the circle shows the amount of wear, which measures ~0.0045" (0.11mm) and is due to slide guide and slide wear causing the slide to be constrained by the needle in the emulsion tube. This results in an excessively rich mixture, affecting predominantly low rpm operation. To put this into perspective, a jet needle base diameter change of ~.0005" (0.01mm)(with a constant height of taper origination) results in a clear change in CO during dyno testing.