AP question

4 replies to this topic
  • jimmydoorlocks

Posted May 03, 2009 - 01:27 PM


I was cleaning the carburetor on my 2000 YZ426 when I discovered that the AP diaphragm was torn. I'm obviously going to replace it but when I go to the parts fiche there are 4 different part numbers for "DIAPHRAGM ASSY AP", and the prices range from $15-$24 at the TT store.

5JG-14940-05-00 1 $21.43 $19.29
5JG-14940-17-00 1 $16.98 $15.28
5JG-14940-18-00 1 $26.68 $24.01
5JG-14940-19-00 1 $26.68 $24.01

My question is: Which one do I get? What are the differences? What is stock on the bike?
Right now, all the jetting is stock and I'll be doing the BK mod before I put the bike back together. Any advice is appreciated!!

  • grayracer513

Posted May 03, 2009 - 06:20 PM


The difference in the diaphragms is the length of the stop button, which controls the stroke of the diaphragm, and thus the total delivered volume.

Read: (from Common Threads)

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  • jimmydoorlocks

Posted May 04, 2009 - 07:08 AM


Thanks Gray, but I've read the stickys and those charts provide exactly zero information that is relevant to my questions. In fact, they don't even list a 2000 YZ426. They list the '01 and '02 YZ426(both of which have different diaphragms), the '01 and '02 WR426 (which have the same diaphragm, but different than both of the YZs) and the 2000 YZ/WR400 (which match the WR426).
I understand that the 5JG-14940-05-00 has a shorter rivet and will pump more gas than a 5JG-14940-19-00, but is that a good thing or a bad thing? Which one of the 4 listed in the fiche came stock in 2000? What are the advantages/disadvantages of of pumping more or less gas with a AP?
I'm not trying to be a pain in the A, but I'm one of those people who likes to know the "why".
Thanks again for the help!! :excuseme:

  • grayracer513

Posted May 04, 2009 - 10:49 AM


I beg your pardon, but if you first note the parts listing from from the catalog, you see 5JG-14940-18-00 listed as "STD" for your bike.

Moving then to the chart in ap3, you find that it has the second longest rivet and thus the second lowest output of the 426 (5JG) diaphragms.

Reading the BK mod, you find that the purpose is to shorten the duration of the pump shot from 1.7 to 0.3 seconds. In consideration of that, I would hardly think that increasing the output of the pump by shortening the button (increasing the stroke) would be productive.

If it were me, I would use the standard part ans perform the BK mod as described, proceeding from there if anything else were required. The BK is a proven winner.

As to how and why accelerator pumps work, when the carb is at idle, fuel flows through the pilot circuit, not the main circuit. As the throttle is opened, air flows through the carb in response to engine intake vacuum and starts the fuel flowing in the main. If the throttle opens rapidly at a low engine speed, the intake vacuum may temporarily drop to a level too low to produce an adequate signal at the main nozzle, and the engine may stumble until air flow is established, or it may stall. Accelerator pumps are intended to mechanically supply the fuel required to run the engine long enough to establish air flow and start the main circuit. They must supply the right amount, however, as too much or too little will cause a stumble as well.

A detailed discussion is here:


  • jimmydoorlocks

Posted May 04, 2009 - 02:20 PM


First, I must have been skimming rather than reading because I completely missed the STD designation in the parts fiche. :smirk: Thanks for pointing that out.
Second, this is exactly the information I was looking for!:excuseme: Thank you.

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