BK's carb mod



10 replies to this topic
  • motoman393

Posted June 19, 2001 - 04:57 PM

#1

Hello everyone,

I am relieved to see that ThumperTalk is actually working again! While the forums didnt work I have been researching and printing out stuff! I have been reading and rereading posts on this mod! I have printed them out and they are very imformative (including the 29 pg one from the WR side LOL)! Has there been any pics taken? If so where can you look at them! If for some reason you do not like the mod can you remove it? For the people who have done this mod...how do you like it? and is it hard to do? If you have any comments or suggestions I would be glad to hear from you! I am starting to seriously think about doing it...but I am a little nervous drilling and tapping a hole in the body of my $700+ carb! Thanks ,

Garrett

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I get my kicks on a 426!
Motoman393's MX Site

  • James_Dean

Posted June 19, 2001 - 05:42 PM

#2

The mod is easy if you know how to drill and tap a hole. The drilling is safe because it is entirely on the outside tab made for it. The screw adjustment can be turned up, down, or completely off. A nice feature for the cost of a screw and spring.

  • motoman393

Posted June 19, 2001 - 06:21 PM

#3

Thank you James Dean...I have read all of your old posts on this subject and you are very knowledgable on jetting! I appreciate your suggestions.

I was reading in previous replies (3 months old) and most people used 4mm x .70 (pitch) 20-25mm long! Do you buy the screw and spring at a hardware store or do you have to order it from Yamaha? The jetting must also be changed to is what I read, right? Thanks,

Garrett

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I get my kicks on a 426!
Motoman393's MX Site

  • Guest_Guest_*

Posted June 19, 2001 - 07:33 PM

#4

As I was saying...

Good to see you're on top of things JD, I have another question for you:

I recently purchased an ’01 YZ 426 (thank you) and I just measured the pump diaphragm button in both my bikes:

’00 – 8 mm
’01 – 7 mm

(all figures give or take)

All the mags said “revised pump stroke,” I assume we all thought that meant shorter, I sure did. But wouldn’t what I found contradict that?

On Parts Fish the cover and rod share identical PNs, side by side the arm and timing screw look identical. What gives?

I suppose I could figure this out myself, both bikes are a few feet away, but all I want to do is ride my new one, I’m tired of working on it. Maybe later...

...after I get my identity back.

[This message has been edited by Hick,nopasswd (edited 06-19-2001).]

  • James_Dean

Posted June 20, 2001 - 06:31 AM

#5

This would sure make the P-38 Lightning a worthless purchase on a '01 YZ426.

The diaphram on stock bikes normally does not reach the pump cover as a stop. The diaphram post height is a little beyond (.5mm?) the normal stroke. If you had set the pump start position to zero delay (or less) it might reach the stop.

The '01 YZ426 stroke may have been shortened by the cam that rotates the linkage. Did you measure the stroke on the rod? That would be the proper way for Yamaha to change it. This was very difficult to measure on my carb. It was somewhere near 3.0mm to 3.3mm on my '00 and ~.115" (2.92mm) on Clark's '99.

Motoman, the screw and spring came from the hardware store. The spring needed to be cut to length. It is nearly the same as a "air screw" spring, P/N 021-235 from Sudco ($6 !!) to be used with a 16mm screw.

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  • Guest_Guest_*

Posted June 20, 2001 - 02:18 PM

#6

JD,

I’m still setting up the ’01, next time I have the carb off I’ll measure the stock stroke length on each, and remeasure the diaphragms, maybe I just goofed that.

Because, short of the diffe

  • motoman393

Posted June 21, 2001 - 07:29 AM

#7

James Dean,

I got a screw/spring combo and a drill and tap! I am going to make the mod next week after our relatives leave! I took the carb off and it looks very easy to do. You said something about moving the needle clip down to make it richer...because if you dont you are only doing have the steps? What would you recommend as a starting point for my jetting @ sea level in Houston TX? I am running basically stock w/162 main, 42 pilot, needle in middle position, fuel screw 1 3/4 turns! I tried the 158 and 168 mains and 48 pilot and it ran better in stock form (I am running a WB R4 pipe)! The lowering the needle helped clean up the "soot" on the plug but it ran better stock. I know jetting varies from bike to bike and rider preference! I was just wondering what a good starting point would be because I read that this mod really changes your jetting! I guess I'm just nervous :) ! Sorry for all the questions...I really appreciate it!

Garrett

------------------
I get my kicks on a 426!
Motoman393's MX Site

  • James_Dean

Posted June 21, 2001 - 10:26 AM

#8

What needle code is in your bike? (OBEJP?)
You mentioned lowering the needle to middle clip, was it in clip #5 stock?
Did you turn the pilot screw in to 3/4-1 1/4 turns when you tried the #48 pilot?

The change is good, but doesn't work miracles. :)

  • Hick

Posted June 21, 2001 - 05:49 PM

#9

Originally posted by Hick,nopasswd:
JD,

I’m still setting up the ’01, next time I have the carb off I’ll measure the stock stroke length on each, and remeasure the diaphragms, maybe I just goofed that.

Because, short of the diffe



Excuse me for talking while you’re interrupting me, but, as I was saying...

Because, short of the different PN and measurements on the diaphragms, and the different colored main spring (blue on the ’01) everything else seems unchanged to me. Side by side the cams look identical.

I’ll figure out what is what at some point, I’m sure y’all are dying to know...

Garrett, if you have an ’00 get the ’01 needle, decent jetting, especially if you have an ’00, is your first priority. IMO the ’01 is darn close right out of the box up here, but in Houston you may want a richer pilot, main and clip position.

  • motoman393

Posted June 21, 2001 - 08:02 PM

#10

Hick and JD,

Yes I have a 01' and the jetting is real close stock (but it could be better)! I was just wondering how much it will change whenever I do the BK mod...I guess I will find out whenever I do it! The needle came in the middle position stock (and that is what I currently have it set at...awhile back I raised the needle clip by 1 groove and it cleaned up the soot but I liked the performance better stock (so I set it back to stock)! I tried the 48 all settings and it ran sluggish (but that was before I got a R4 pipe) maybe it will run better since it is "opened up" ! Thanks,

Garrett

------------------
I get my kicks on a 426!
Motoman393's MX Site

  • James_Dean

Posted June 22, 2001 - 12:30 PM

#11

Test ride it in clip #4 (where you're at currently) and clip #5. Same pilot setting you're at now or slightly richer.




 
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