BK's Carb Mod

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#1
Does anyone know if the accelerator pump mod is only for the '00426, or both '00 and '01?
 
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#2
The mod is to limit the pump stroke, but I don't know if it applies to the '01 yz426.
 
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#3
It won't work on the Slant, but does apply to the '00 and '01. The '01 just needs it less since it has a shorter pump stroke than the '00.

IMO an adjustable accelerator pump is a must-have mod on the 426.

Get out yer drill...
 
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#4
Hick

Thanks for the reply. I did a search on TT (BK) and found many posts.
Did you see the pictures I posted?
(TT yz gen discussion, "photos") Also gen. discussion, Fire!
 
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#5
Yeah, I saw those pics., but if you posted a shot of the burned YZF I missed that. Sadly, TTalk is down right now. I just get an empty forum the last few times I have checked.
 
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#6
Hick

The burned bike pic's are on DRN, General discussions, Fire!
Also, I have been doing a lot of jetting work, ( you wont believe the spec's), and I just took the carb off and measured the acc. pump squirt at 1.7 sec's. I think "the screw" is in order. I can't believe the reccomendation is .3 seconds. This seems like such a big step from 1.7 seconds. Look out drill press!
 
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#7
For those following at home, I used a hand drill, but a drill press, in the right hands, is probably better. You want to be careful anytime you drill holes in a seven-hundred dollar carb. I used the same spring found on the screw that adjusts the pump timing (PN 5JG-14531-00-00) for the pump stroke adjuster.

I’m sure it wasn’t too hard, but I don’t know how I would accurately time the duration of the “squirt.” James Dean, the master carb tuner of cyberspace (so far as I can tell, anyway) ended up with 1 mm of stroke, measured at the rod. Just remove the bowl, measure how much actuating rod you have protruding below the carb body with some calipers, hold the slide open and measure again then subtract for the pump stroke.

When I originally did this I turned the screw all in and then jetted the bike, I think you should jet with the pump off anyway, then I just turned out until I was happy with the response. Once I read on TTalk what James Dean came up with I measured what I ended up with and it was damn close to 1 mm as well, so you may just elect to start from there.

I loved what this did for my ’00 so much that I did the same to my ’01 before I even rode it (I should’ve taken off that $%&# front tire too).

Ricky, I don’t know how I missed that Fire thread, that sucks. I know a guy who’d have bought that brake pedal, it looked salvageable...

I have also noticed the occasional scent of singed Mesquite and Creosote bushes off my header but I guess I never imagined the potential implications...
 
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#8
Hey Hick,

I briefly glanced over this subject at Thumpertalk...and I thought to myself, "that is unecessary and not worth it"! But after reading this post and reading and printing out the 3-4 threads from Thumpertalk (yes the old posts still work :) ) I think that this sounds like a great mod!

Is there any special hints or things that didnt go smoothly! I am very mechanically inclined (have split cases/topends/fork seals ,etc,etc) and I do not think this will be all that hard? and I was wondering if you have any tips or tricks? Thanks, (off the TTalk to read about BK's carb mod agin)

Garrett
 
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#9
Hick and Motoman

Hick,thanks for the help. I put the carb in a vise, used a drill press, then without moving the carb put the tap in the drill press and rotated the spindle by hand to tap the threads. Worked great.
The duration of the squirt was 1.7 seconds, stock. BK reccomends .3 seconds. I was able to time this with a stopwatch. Open the throttle all the way,and start the timer at the same time, while watching the fuel squirt. When the fuel stops, stop the timer.
Don't forget to set the timing of the squirt. The fuel should not hit the slide. Use the adjusting screw on the cam.
I rode it last night after installing the screw. What a difference! I know i still have to re-do the pilot jet and the needle, but still I almost looped the bike several times! Like Hick said, this is a must-do mod for the yz426.
 
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#10
Rick, Could you post a picture of the finished mod? I've been interested in this mod since BK posted it on TT, but I had already installed the factory R&D
P-38. Do you guys think I would benefit from this mod while leaving the p-38
installed?
 
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#11
Dirtdad

I don't have a way to post a picture, but it really isnt needed. If you take the plastic cover off the right side of the carb, you will see a black plastic cam, that pushes the pump rod down. The bottom left part of the cam moves back, towards the rear of the bike. It moves toward a half moon shape tang on the carb body. Drill a 1/8" hole in the tang, use a 4mm x .70(thread pitch), tap, and a 4mm x 20-25mm screw or bolt with appropriate spring.

DIRTDAD
Before you do this I suggest that you take off the carb, and look at/check the duration of the squirt. BK reccomends .3 seconds. I am really interested to know of the duration with the p-38.

Another benefit is that the acc. pump is now highly adjustable.
 
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#12
Thanks for the info Rick. My son and I are going on a riding trip tomorrow and will be gone for about a week. When I get a chance, I'll try and measure/time the pump stroke or duration with the p-38 and let you all know what I find. I may wait til I get back though. Thanks again.
 
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#13
Garett,

No special tips or unforeseen snags, like ricky1 said it is very simple, the hardest part is mustering the courage needed to drill a hole in your carb.

Dirtdad,

I know that the P-38 shortens the stroke relative to the stock ’00, I’m not sure about the stock ’01, but I doubt it shortens it as much as we are talking about. I think I remember someone on TTalk (probably Clark Mason) figuring this out, but I also remember that Factory R&D made more than one size (maybe just two, one WR and one YZ model).

A tip for ’00 owners who don’t like mixing power tools and their carbs is to get the stock ’01 pump diaphragm, only around $16 bucks this probably works almost as well as the P-38. While you’re at it swap the ’01 needle if you still have the stock ’00 part in there. That could be the best $30 you ever spent on your bike.
 
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#14
Hick, I just recently installed an 01 needle in my 00. I'll be testing it out in the next couple days. When I ordered the part from NCY, the parts guy even mentioned that this has been a popular upgrade for 00 owners. I look forward to seeing what difference it makes. I've been hesitant to do the BK mod cause I thought the p-38 was a sufficient solution. Of course it's like most things. You don't know if something is better unless you try it. Rick pointed out the obvious in that if anything, the BK mod provides a greater range of adjustability, but once you have it "dialed" why would you make any more adjustment unless you make some big change to the motor. I guess I just answered my own question in my reference to being able to "dial" it in.
Thanks for the info.
 
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#15
One question...where do you get the 4mm screw and spring? I heard you need to order another pilot screw or something (order from yamaha)? Or can you get it at a local hardware store? Sorry for all the questions...I just want to know exactly what I need and order it before hand, so that I can ride the next weekend! Thanks,

Garrett