BK Mod Questions

[YZThumper]

I just finished installing and testing the BK Mod on my 2000 YZ 426. The mod was relatively simple to install, but the results were not overwhelming. I believe the bike revs a little quicker and cleaner, but I have not completely removed the hesitation when wicking the throttle.

I am able to get at the new spring/screw I installed to adjust the pump spray duration without taking the carb off the bike. I initially set my duration to around .5 seconds (.3 is what Brian Kinney says is optimal) and put the carb back on the bike. My stock duration was probably at or around 1.5 seconds. When I first started the bike and took it for a spin, I noticed very little change. I came back and turned my new adjustment screw in 1/8 of a turn and rode again. I can feel the response getting a little better, so I adjust twice more, an 1/8 of a turn each time. I can now feel a noticeably better throttle response and the hesitation off bottom has been significantly reduced but it has not disappeared completely. With each subsequent 1/8 turn I can no longer feel any gain in performance. When I took the accelerator pump cover off the bike, I noticed my adjustment screw was against the pump cam, in effect I had disabled the accelerator pump. I didn't notice any major difference in performance during my last spin, so how do you know when your accelerator pump is working or not ?? I backed the screw out a few 1/8's until I found my peak performance again and left it there. How do you know when the accelerator pump is not working and what effect will this have on the bike ?

At one point in my garage I left the bike idle on the stand while I was adjusting the new screw and at the same time I was wicking the throttle on the bike to test response. I may have been doing this for 2-3 minutes, and the next thing I know I have anti-freeze coming out the over-flow. Does reducing the amount/duration of gas from the accelerator pump cause the bike to run lean and therefore too hot ?? Or was it just that I was wicking the throttle too long while the bike was idling and standing still ??

I've also heard people say to try the 01' Needle in the 00 Model. How is the 01' needle different than the 00 model ?? Has anyone tried this by itself or in combination with the BK mod ?

 

[Kramer]

I have a '98 so I can't relate any experience with the BK mod, but you overheated chiefly because you were running the bike still too long. No wind = very little cooling for that motor.

Completely eliminating the hesitation is alot to ask of any four stroke.

You will probably have to go back and forth between fuel screw and BK screw to tune it the best possible. Make sure you have fresh gas too.

 

[Hick]

Originally posted by YZThumper
I've also heard people say to try the 01' Needle in the 00 Model. How is the 01' needle different than the 00 model ?? Has anyone tried this by itself or in combination with the BK mod ?

The ’01 needle is richer than the ’00 needle just off the pilot circuit, and a half clip richer everywhere else. On a tip from James Dean I tried this in my ’00 not long after I bought it, before I touched my accelerator pump. It is much better, IMO that needle has to be the best $12 dollars you can spend on the ’00 YZF.

But if you want to explore this I recommend another post, put the words “YZF,” “Jetting,” in the title and all the pertinent details in the post and you may scare up JD, then your jetting questions will be attended to in much more detail (and w/ greater accuracy). A lot of guys end up richer on the clip position (regardless of what needle they run) after shortening the pump stroke but I did not.

As for your issues with the pump mod, my only advice is to reset the stroke and check the timing and operation of the pump to make sure everything is working properly.

I set the stroke (duration) on mine initially by measuring the travel of the actuating rod (seemed easier than trying to find a stopwatch) with a pair of calipers and the bowl off the carb. Stock was about 3 mm, I think half that is a good starting point. With the slide held open, simply turn the screw until the rod has lifted 1.5 mm from where it was stock.

After you set that I recommend you check the pump timing (the OTHER screw that touches the black arm, but facing towards the rear of the bike), make sure the spray JUST misses the slide as it opens (turn the screw in to delay, out to advance the spray relative to the slide). I think you should try additional delay until you like the response, THEN try more (or less) pump stroke. That is what worked best for me, on both my ’00 and ’01 I ended up with additional pump “delay” to the tune of ¼ to ½ turn over stock.

It is interesting to note that the timing screw AND the tab cast into the carb body for the “BK” screw are both new for the Yamaha Flat CR (’00 and newer), neither appear on other Keihin FCRs.

Hope this helps.