Jet Keihin Pilot #40 (FCR-MX) (4/Pkt)

The Keihin Pilot Jet #40 is a direct-fit replacement slow jet for the Keihin FCR-MX flat-slide carburettor series, widely used across motocross, enduro, and off-road four-stroke motorcycles. Sold as a pack of four, it suits riders who tune their jetting across different altitudes, temperatures, or fuel types and want spares on hand.

The pilot jet governs fuel delivery at idle and low throttle positions — a critical tuning point for crisp off-idle response. Having the correct pilot jet size dialled in prevents hanging idle, flat spots, and poor low-speed fuelling. Size #40 sits in the mid-range of the FCR-MX pilot jet spectrum and is a common starting point for many stock displacement bikes running near sea level.

• Genuine Keihin replacement pilot jet for FCR-MX flat-slide carburettors
• Size #40 — meters fuel at idle and low throttle positions
• Pack of 4, ideal for jetting experiments or keeping spares in the workshop
• Designed for easy swap during carburetor tuning without tool damage
• Compatible with the FCR-MX series used on many motocross and off-road four-strokes

• 4 x Keihin Pilot Jet #40 (FCR-MX)

Install with a flat-blade screwdriver sized to the jet slot — apply only finger-tight plus a snug quarter-turn; overtightening will damage the soft jet body and carburettor body threads.
Remove the pilot jet by accessing the carburettor float bowl; always work with a cold engine and ensure fuel is off.
If reusing a jet, clean with dedicated carburettor cleaner and compressed air through the jet orifice only — never use wire or drill bits to clear the orifice, as this alters the calibrated size.
Inspect the jet orifice and threads for damage or corrosion before reinstallation.
When tuning, change one jet size at a time and test ride between changes to accurately identify the correct size for your altitude, temperature, and fuel.
Store unused jets in a sealed container away from moisture and solvents to prevent corrosion of the orifice.
Keep a jetting log noting jet size, altitude, temperature, and bike response — this simplifies re-jetting for future trips or season changes.
Dispose of worn or damaged jets rather than attempting to repair them; the calibrated orifice cannot be reliably restored once altered.