boyesen accel. cover vs. bk mod

2 replies to this topic
  • 800screws

Posted September 14, 2005 - 01:12 PM


do they do the same thing? on my 02 426, i richened the jetting for a fmf t4 silencer since it was backfiring went letting off the throttle, now in some tight sections, it doesn't have the snap right off the bottom like it used too. i am caseing the second double on a double double section that used to be no problem. would the accel cover fix this or should i just go in on the fuel screw. when i had the fuel screw at stock postion the pipe would glow red. it's out 2.5 turns now and it runs great except for a situation like i mentioned before.

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  • YZViper

Posted September 15, 2005 - 12:50 AM


First thing first. Check your squirt duration before you spend the money on the Boyesen or drill into your $900 carb. Mine from the factory seems to be right around a second or so.

Also do the JamesNow mod. Unreal what a difference it makes on the low end snap.

I am 2 turns out and JamesNow mod, no problems with a double double situation.

  • Scotty426

Posted September 15, 2005 - 01:33 PM


The Boysen people will tell you the cover is about the quality of the accelerator pump squirt (IE no bubbles). Where do the bubbles come from? I have heard slow fill characteristics and a pressure differential when you are on and off the accel pump constantly (woods riding). The holes that allow in gas to the reservoir are placed differently and the depth that the diaphragm can go down to is different. I had a hot stall problem that I felt like the cover cleared up.
The BK mod will allow you to adjust squirt duration. Mine was stock with a 2 second plus squirt, by experimentation I have adjusted to just over a second.
One more idea. I called Boysen and they e-mailed me instructions to install a leak jet like the new YZs have . The leak jet leaks some of the accel pump pressure back into the bowl. This decreases the pressure of the squirt and of course the volume delivered over time. It was cheap and easy. Mine liked a #58 I think. (stock on a KX250F). I don't remember for sure. The result of these cheap mods was a woods friendly YZ 426 with crisp response. I am very happy. Mine did not like the quiet core insert though. I never figured that out. I am unfamiliar with the JAMES NOW mod and can not comment.

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