Help Please! confusion regarding Keihin FCR MX39 and BD Mod, Taffy Mod, O-ring Mod


I have a 2002 YZ426F and it has the stock carb installed which is the Keihin FCR MX39.

I have been reading many posts and threads about the above mentioned mods and am confused about which ones are applicable to my bike, and can/should they be done together.

I have a bit of hesitation or bog when getting on the throttle and if the O-ring mod will help this it will make me happy as it is the easiest mod to do.

Any insight into what mods are applicable to my carb and which one/ones will help with the bog would be appreciated.



I rode a 2000 YZ426 for eight years in completely stock condition, and never had any issues with it. Get it jetted properly, and you should have no problems with it.

The first thing to do is to visually check the duration and timing of the fuel squirt. Ideally you want the squirt duration to be about one second and to just miss the slide as it lifts up. If the duration is more than 2 seconds, you need to limit it using the BK Mod or an AP diaphragm with a longer pin. Also try the O-ring mod which increases the force of the squirt. When you can, do a 'Search' as there is lots of info available on this topic.

The Taffy mod is for YZ400's only.

The BK mod applies to the 426. It should fix the problem all by itself.

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