01 426 off idle bog

5 replies to this topic
  • Motorod

Posted January 07, 2003 - 10:49 AM


I had an 01 426 and it had a hesitation off idle. The bike threw me in a 6 pack (3rd gear) set of large loops, when coming off the second set of woops I gassed it and the hesitation occurred causing me to face the next whoop and throwing me over the bars. Didn't get hurt, but traded the bike in on a RM250 the next week. I wreck enough without the bike helping me like that. I heard the 02 426 fixed this problem, but I keep hearing you guys talking about the BK mod. Was this mod supposed to fix this problem or is it for something else, or was the problem fixed when the 02 came out? Just curious. I loved that bike except for that problem.

  • Hick

Posted January 08, 2003 - 07:57 AM


Your bike had a jetting issue so you sold it? Since I live at altitude I can't afford to do that, every bike I've owned needed to be rejetted, four stroke or not.

As for your question the accelerator pump diaphragm was changed from '01 to '02, it is a pretty safe bet that this change resulted in a shorter accelerator pump stroke, but not as short as 99% of the guys on this board who did the BK mod end up with.

But without riding your '01 it is hard to say if the different pump diaphragms or BK mod would have helped your problem. If you are lean on the needle or clip position that will really make the bike fall on its face when you grab a handful of throttle.

Having said all that if you are talking about a WR all bets are off. Because of the way those bikes are delivered (tiny tiny exhaust insert, sealed air box and throttle stop) the jetting is pretty weird. Removing those restrictions will obviously change the jetting significantly. Probably nobody leaves that stuff on, and I've ridden three such WRs with stock jetting and they all ran terrible (but then I own a YZ, so it was obvious to me).

I convinced all three owners to let me change to YZ jetting (more or less) and all three were amazed at the throttle response (I also did the BK mod) and how much better/stronger it ran altogether.

So IMO any WR will need to be rejetted to run its best once you take all that crap off.

The BK mod:

Starting in 2000 Yamaha redesigned the Keihin FCR 39 to better suit them, 426/400s from '00 on have this proprietary version of the carb (called the Flat CR). It is equipped with a tab cast into the carb body where a set screw can be placed to adjust/limit the accelerator pump stroke, all you have to do is drill and tap the tab and install a spring and a screw. Tim Ferry's old mechanic (Brian Kinney) came on Thumpertalk one day and mentioned this mod, his Thumpertalk handle is BK, so that little screw installation came to be known as the BK mod.

Hope this answers your question(s).

  • Motorod

Posted January 08, 2003 - 09:34 AM


At that time I had a 01 426F, not a WR. I talked to everyone that had this kind of a bike, and all had this problem. I new I had the problem before the wreck, but did not think I would get into that kind of a situation. No one I ask at the time new how to fix this problem, including my dealer. I did not now of TT at the time. I'm still not clear on your answer, would the BK mod have corrected this off idle hesitation? My alt. is 800 ft.

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  • tctrailrider

Posted January 08, 2003 - 12:30 PM


Yes, that would be the reason for the BK mod. Stock the fuel squirt is between 1 and 4 seconds and seems to vary alot bike to bike. Most set 1/2 second or less with BK mod. With the BK mod and proper jetting you can make it jump.

  • cnacc

Posted January 08, 2003 - 01:15 PM


Hick, come on out and visit. I'll show you elevation. :)

  • Hick

Posted January 08, 2003 - 05:36 PM


I'm still not clear on your answer, would the BK mod have corrected this off idle hesitation? My alt. is 800 ft.

I guess my answer is I can't really say, what you described really sounds to me like it could have been lean on the needle clip. It seems to me that when a two stroke is a clip lean on the needle it may surge or lose a little power, but a four stroke one clip lean will really fall on its face.

One thing is for certain and that is the BK mod (properly adjusted) certainly wouldn't have hurt.

Hick, come on out and visit. I'll show you elevation.

:D Yeah, no doubt huh? I've ridden my 426 at elevations up to 9,000 in New Mexico (Sacramento Mts. are just a few hours away, great trails) and I've been to Rampart near Denver (8,500 or so??). That place is too much, although I'm glad we got to ride during the week, I was scared of having a head-on over the weekend with all the other riders (all of a sudden I was reluctant to ride point :) ). I wish I lived closer, on the other hand I wouldn't trade my riding areas with anybody (that I know of).

I have a couple of real close friends in Denver, both ride, at some point I'm going back for some more Colorado riding. Right when it starts hitting 100 degrees down here that will surely move up my priority list...


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