426 Won't Open Up

6 replies to this topic
  • mil81

Posted May 13, 2013 - 01:36 PM


I am working on my Uncle's Yamaha 426 and am running into a brick wall.

Bike is an '01 426 with an adjustable cam and FMF exhaust

The bike starts like a dream, idles great, runs quite well until about 1/4 throttle. That is where is stops opening up. 1/4 throttle to full throttle there is no change in rpm. It actually "bounces" there like a tuner car hitting a rev limiter. When I pull the air cleaner off it stops "bouncing" but will still only run 1/4 throttle (Maybe 3000-4000 rpm). The plug is a dual electrode which I am not accustomed to reading, but from a short plug shop at full throttle the center electrode is a little bit white. Could a lean issue be causing this? I can verify jetting next time I have the carb apart.

The valve timing has been verified by two professional shops. Leak down test came back at 98%. Has new spark plug. Carbs have been cleaned many times over...

Any ideas?
Thanks a lot for your help

  • grayracer513

Posted May 13, 2013 - 03:53 PM


Check the lead that runs from the neutral switch to the CDI. If it's grounded anywhere along the way, the CDI will think it's in neutral, and the rev limit will be lowered to around 6500.

Otherwise, you might check the stator for damage such as loose coils or damaged wiring, and the carb for a dropped needle, or a damaged or incorrectly installed vacuum release plate on the slide. The "square" side goes down.

  • mil81

Posted May 13, 2013 - 09:01 PM


Awesome. Thanks for the advice. The plate on the slide is oriented correctly, and everything else appears ok. I will check the electrical soon and report back

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  • mil81

Posted May 14, 2013 - 06:43 AM


I believe the neutral has been eliminated. Which wire is it out of the cdi?

  • grayracer513

Posted May 14, 2013 - 06:58 AM


"Sky blue"

  • mil81

Posted May 14, 2013 - 07:33 PM


Beauty. That wire is still plugged into the cdi, so I will pull in out of the plug tomorrow and see if the problem goes away

  • mil81

Posted May 19, 2013 - 07:40 PM


Hmmm, so when I disconnect the sky blue wire from the grey 6 wire connector to the cdi the bike won't run at all? I am positive it wasn't grounding out. It just plain won't run without the wire. Any suggestions?

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