Jetting for a 400-426 conversion


7 replies to this topic
  • SDBSKI

Posted December 01, 2010 - 07:38 PM

#1

I'm converting my '98 400 to a 426 and using my 400 carb. I'm running a Hot cams exhaust cam and a white bros E series. Foam filter in a stock YZ box.
I usually run at about 1500-3000 feet elevation.
I was thinking of starting out with stock 426 jetting, but the carbs must be different because the 426 has smaller stock jets.
Does anyone know of a good baseline to start with?

  • grayracer513

Posted December 01, 2010 - 07:52 PM

#2

The 400 has a totally different carb. Don't use 426 info to jet it. Start with exactly what your 400 had in it, then go from there. This post will help:

http://www.thumperta...699#post2881699

  • SDBSKI

Posted December 02, 2010 - 06:32 PM

#3

Thanks! Some good info. I believe i am a bit lean already with the 400 at mid range after reading that post. It would miss at low throttle cruising.

  • 250stroker

Posted December 04, 2010 - 04:52 PM

#4

hi, im new to thumpertalk and i;m wondering if your 426 motor bolted right up in your 400 frame. how about the electrical? my 400f blew and i cant find a new motor and the damage to my 400 is way too $$$ to fix. thanks!

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  • SDBSKI

Posted January 14, 2011 - 04:03 PM

#5

I 426 uses all the same externals as the 400. The swingarm bushing might be the only issue.

  • SDBSKI

Posted January 14, 2011 - 04:04 PM

#6

How much larger should I go on the pilot and main as a good starting point for the 426 with 400 carb?

  • grayracer513

Posted January 14, 2011 - 04:35 PM

#7

You should discard the 400 carb, or at least make a paperweight or mobile of it. The carb from a 426 is much more sophisticated, and the carbs from an '05+ YZ450 even more so. They bolt right in place, too, with only minimal mods. You won't have a hot start with the 400 carb, either.

Either way, start with the jets already in the carb and let the motor tell you which way to go with it.

  • SDBSKI

Posted January 18, 2011 - 08:05 PM

#8

Thanks gray. I'm going to see how the 400 carb does before I discard it. The hotstart port is built into the yz400 head that I'm using with the 426 cylinder so it will still be functional.





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