Keihin FCR Accel Pump Diaphragm Install Question - QUICK!

8 replies to this topic
  • fadingfastsd

Posted September 13, 2007 - 10:51 AM


Hey guys, I've got my carb torn down on my 99 WR400 installing a ZipTy adjustable fuel screw, bigger pilot jet to get rid of the backfiring/popping on decel and crappy idle (its got open air box and FMF exhaust, pilot was never rejetted for it, only main jet), and I'm installing a new accelerator pump diaphragm b/c the old one was cracked and brittle.

Which way does the diaphragm install into the float bowl. I've taken pictures showing it both ways, can you guys tell me which is correct? The shop manual does not show a clear picture of it, so I'm not sure. Thanks!

#1 (with the 4 spaces in the metal ring)
Posted Image

#2 (with no spaces in the metal ring)
Posted Image

And also, did I install the plastic piece that fits around the main jet correctly?
Posted Image

A quick response is much much appreciated as I have it torn down right now and want to get it back together soon!



  • jbrooks26

Posted September 13, 2007 - 11:35 AM


The long part of the rivet should be showing or pointing into the AP cover. As for the plastic cover, It is hard to tell if it is right from that picture, but I think it can only really go on one way. Hope this helps.


  • fadingfastsd

Posted September 13, 2007 - 11:47 AM


So picture #2, with the longer center part of the diaphragm pointing INTO the cover? Is that center pin just the stop for when it hits the end of the cover?

  • jbrooks26

Posted September 13, 2007 - 12:01 PM


Picture number 2 would be correct. The center pin (rivet) length determines the maximum amount of fuel that can be pumped when the AP actuator shaft pushes on the diaphram to pump the fuel. The shorter the rivet the more fuel the pump can move. I hope this helps,


P.S. I did some searching and it looks like your plastic cover may be upside down.

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  • fadingfastsd

Posted September 13, 2007 - 12:28 PM


Hey thanks for the reply. The cover has to be installed correct, b/c if it was the other way it would hit the float bowl and not allow it to seal. I installed the diaphragm like #2, put it back together. The bike fired up in a couple kicks, idles like a champion, and my popping and backfiring on decel are gone! The bike is running amazing!

I'll post my specs in the jetting thread, but for anybody reading this, with an open air box, all the free mods done, and an FMF Power Core4 muffler, I'm running a 48 pilot instead of the stock 45.

Thanks guys!

  • jbrooks26

Posted September 13, 2007 - 12:32 PM


No problem, glad to help.

  • tntmo

Posted September 13, 2007 - 02:54 PM


Good to hear that your bike is running right now. It wasn't too difficult of a fix either. Let's ride!

  • maxtherat

Posted September 13, 2007 - 09:00 PM


What are you running for the needle? I have gone through 2 of those diaphrams in 3 years. I do run 50% race fuelVP u4. Would that do it?I'm suprised that you 400 comes stock with a 45 PJ. My 01 426 uses 42 stock.

  • Silverslug

Posted September 14, 2007 - 05:52 AM


U4 will do it, it is very corrosive or as I was told it will eat your gaskets and seals. They recommend it to be drained out after every ride. I use VP 110 you don't have to drain it.


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