Jetting for Keihin FCR

The most complete app for setting up Keihin FCR carburettors. Simulate jetting setups and compare them across the whole throttle range, for motocross, enduro and supermoto bikes and any four-stroke running an FCR.

It covers all three variants: the flat-slide FCR-MX fitted as standard to 250F and 450F bikes, and the downdraft FCR-D and horizontal FCR-H. All three share the needle tables, and the series —N427-0C or N427-90— is chosen by the carburettor BORE, not by the family: that is why the app asks for both.

The idea is simple: mark the jetting you know works as your reference, and the app tells you what to change when conditions change, and what every change does to the mixture.

WHAT YOU CAN DO
• Create as many setups as you like, each with its name, colour, notes and date.
• Read the AFR against throttle chart: lean at the top, rich at the bottom, with every setup drawn on it.
• Compare each setup with the reference over three ranges: low (0-25 %), mid (25-75 %) and high (75-100 %), with the difference as a percentage.
• Duplicate a setup to try a change without losing the one before.

THE WHOLE CARBURETTOR
• Main jet and pilot jet.
• Needle and clip position —seven on the FCR—, with the needle's own dimensions on screen.
• Fuel screw, in turns out from the seat. Careful, it works the other way round to a two-stroke air screw: opening it richens.
• Slow air jet: same range as the pilot jet but the other way round — a bigger one leans the mixture.
• Carburettor family and bore: FCR-MX from 32 to 41 mm, FCR-D and FCR-H from 28 to 41.
• Fuel density.

NEEDLES
• A catalogue of Keihin needles with their geometry: tapers, lengths and straight diameter.
• Search by needle code or by bike make and model, to find which needle came as stock.
• Needle analysis: pick two, each with its clip, and compare both profiles on one chart.
• Tell the app which needles and jets you actually have on the bench, and the lists will only offer those.

RECOMMENDED JETTING FOR THESE BIKES
Pick the bike, the year and the conditions and the app proposes a setup, ready to create and fine tune:
Honda CRF450R (2002-2008), CRF450X, CRF250R (2004-2009), CRF250X (2004-2019) —stock and uncorked— and CRF150R (2007-2024), with both exhaust variants; Yamaha YZ250F and YZ450F (2003-2009), WR250F and WR450F; Kawasaki KX250F (2004-2010) and KX450F (2006-2008); Suzuki RM-Z250 (2004-2009) and RM-Z450 (2006-2007), and KTM 250 SX-F (2007-2010).

Plus the stock carburettor of 38 more models, so the reference bore is the one your bike came with: besides the ones above, Yamaha YZ400F/426F and the YFZ450 quad; KTM EXC Racing 250-525, SX Racing 525, EXC 400-530, SX-F 250-505, EXC-F 250, LC4 625/660; Husaberg FC/FE/FS 450 and FE 501; Husqvarna TC/TE 250 and TE/SMR/TC 450-530, and Beta RR 4T 350-525.

WEATHER
• Temperature, humidity, pressure and altitude, by hand or from the nearest weather station.
• Air density and density altitude worked out as you type.
• Metric or imperial units: °C/°F, mb/inHg, m/ft, kg/m³.

HOW TO USE IT
1. Build your starting setup with the engine, the carburettor and the weather of the day.
2. When you find one that works on track, mark it as the reference and write down how it behaved.
3. From there, create alternatives to see what changes when you go up in altitude, when it gets colder or when you touch a jet, and take the one that adds up to the track.

ALSO
• Help on every screen, explaining what each setting does and which way it moves the mixture.
• In English, Spanish, French, Italian, German and Portuguese.
• iPhone and iPad. Works offline, except for the weather station lookup.

The model is empirical and meant for comparing setups with each other. It is no substitute for track testing or for reading the spark plug.

An independent app, not affiliated with Keihin or Hitachi Astemo. Brand and model names are used only to identify parts and engines.