01 YZ 426 diagnosis

6 replies to this topic
  • G-BRO

Posted April 21, 2014 - 10:42 AM


Can't seem to pinpoint whats wrong with my 426. Did a top end (piston, rings, valve springs, shims, cam chain, hotcams ex auto cam) and carb clean about 20 hours ago. Bike ran great the first few rides. 


The last 3 hours or so. It will always start first kick with or with the hot start, it will idle great when cold and usually for about 20 seconds or so when hot (until it erratically stumbles and dies). It runs well when you aggressively open the throttle from any position, but has a constant stumble from idle to about 1/2 throttle once you start rolling back on the throttle again. This trend is very consistent once hot. It is not a "bog". The more aggressively you ride it the worse it gets. 


I have cleaned the carb, changed all the jets and settings multiple times and have still not resolved it. Everything seems to be installed correctly, I've tried a different gas cap and vent, cleaned the petcock, changed and checked the spark plug and checked all the electrical connections. 


Any ideas or suggestions? I really don't wan't to tear it down too far again unless I have to. 


Float height? Air leak? Bad CDI? Bad or improperly worn cam??


Its extremely annoying 

  • G-BRO

Posted April 21, 2014 - 11:00 AM


Also, I believe the vacuum plate on the slide is installed correctly. It should be viewed as an "M" when installed not a "W" correct? The hole being closer to the bottom?

  • grayracer513

Posted April 21, 2014 - 12:16 PM


Correct on the slide.


The bike  sounds rich: Idles when cold but not well when warm, etc.   Smaller pilot or correct adjustment is likely the cause, or a float level set too high. 

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • G-BRO

Posted April 21, 2014 - 01:22 PM


What I don't understand is that the bike ran great before with the same pilot jet (42). I tried both cleaning the 42 and installing a new one with no change. Is the float level is very touchy or does it have to be pretty far off to notice a substantial difference? It visually appeared normal, but I will remove and measure with the book. 


Also- I wasn't able to find the small o-ring for the pilot screw when I removed it. Must have fallen out. Not sure how much this may play a factor. 

  • grayracer513

Posted April 21, 2014 - 01:43 PM



Also- I wasn't able to find the small o-ring for the pilot screw when I removed it. Must have fallen out. Not sure how much this may play a factor. 


Yes, let's leave out the important facts until the end.


Replace the O-ring and try again.

  • G-BRO

Posted April 21, 2014 - 01:52 PM


Ok, I'll try that first. Appears that the seal on the vacuum plate is "waffled" around the edges as well. Looks like I can get an OEM one online, but do you know if its easy to put them back on correctly? Looks like you may need to stretch them a bit to fit it correctly.  

  • grayracer513

Posted April 22, 2014 - 06:12 AM


Try leaving the one you have out on the bench overnight.  It will probably return to its correct shape as it dries.  Not unusual.

Related Content


Michigan Motocross Tires by 288yz450

Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply

yz400f throttle bog by yz400fmaniac

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • * * * * *
  • Hot  28 replies

Help with 2000 yz426 by weekend woods racer

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  36 replies
WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info

  • 0 replies

Need help by Goforaride

Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  68 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.