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mdkcrf250r

BK Mod on 08 YZ450F?

3 posts in this topic

I love my 08 and I'm pretty sure I have my fuel mixture screw dialed in pretty good but sometimes when snapping the throttle open quickly the bike flames out. I'm suspicious that the accelerator pump may be pumping for too long and maybe not even at the right time. Is the BK mod possible on the later YZ450F's? Has anyone here done it on an 06+?

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The BK is only applicable to a 426. The FCR-MX on the '08 (and '06 and up) has a completely different accelerator pump circuit than the 426. The fault, when there legitimately is one, is usually too small a pump shot, not too big a one.

Before I say more, I'll tell you what many here know already, which is that I am not an advocate of beefing up the AP until the engine can stand any kind of instant snap of the throttle that any goon rider comes up with. The overall tuning of the engine suffers in other areas when this is done. A good rider should be able to adapt to "rolling" the throttle and never notice that the bike did not respond essentially instantly.

However, if you really think that the bike bogs too easily, and sometimes they do, here are a list of tuning fixes available with your carb:

  1. A needle with a smaller major diameter (see the tuning section of your manual)
  2. A smaller leak jet
  3. A stiffer AP linkage spring from Merge Racing
  4. A pump diaphragm with a shorter limit button

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I've got a 08 and before I got to radical I would pull the pilot jet and make sure it's nice and clean.

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