KTM 640 LC4 Mikuni BST40 emulsion tube

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 dynamometer testing.

slide guide, new

slide guide, used

slide, new

slide, used

slides, new and used LC4

jet needles, new and used

New BST40 emulsion tubes can be purchased here.

Applications list:
KTM 400 LC4-E '01
KTM 400 LS-E/mil '04-'05
KTM 400SXC '00
KTM 625SXC '03 (USA only)
KTM 640 Duke II '00-'07
KTM 640LC4 '98-'99 & '02
KTM 640LC4 Adventure R '99-'07
KTM 640LC4-E '00-'01
KTM 640LC4-E/mil '98
KTM 640LC4 Enduro '03-'05
KTM 640LC4 Supermoto '01-'05
KTM 640LS-E/mil '00