PROX N424-21 Keihin Pilot Jet #42 Honda CRF150R

The Pro X N424-21 is a #42 pilot jet machined specifically for Keihin carburetors fitted to Honda CRF150R dirt bikes from model years 2007 through 2009. The pilot jet is one of the smallest but most consequential components in a carburetor's fuel circuit — it governs fuel flow at idle and through the low throttle range, directly affecting how the bike pulls from a stop, idles on the stand, and responds during slow technical riding. A worn, clogged, or incorrectly sized pilot jet shows up immediately as a hesitant or erratic idle, lean stumble off the bottom, or a persistent need to fiddle with the fuel screw to keep the engine happy.

Pro X manufactures this jet to tight tolerances appropriate for Keihin carburetor thread and orifice geometry, meaning it installs cleanly and seals correctly against the float bowl without requiring thread chasing or sealant. The #42 size places it in standard pilot jet territory for the CRF150R application — though riders who have modified intake or exhaust components, or who ride at significantly different altitudes than sea level, should cross-reference this size against their specific tuning needs. Installation is accessible to any rider comfortable pulling a carb off the bike: drain the float bowl, locate the pilot jet on the carb body, swap it with a flat-blade screwdriver, and reassemble. Note that this part is not available to California residents per CARB regulations.