426 Jetting

2 replies to this topic
  • smdonald

Posted June 06, 2011 - 06:32 AM


I've been through all the stuff I can find on jetting in the sticky's and also search results. Im stuck once again. Thank God for TT though!

I've got a 2001 YZ 426. Top and bottom end are new. Stock pipe for now. Bike starts easily but will only idle w/ the choke on. As soon as choke is released, it will die immediately, even if i try to rev it up w/ the throttle. Ive riden it very little, just up and down the street but it has to be w/ the choke on. Im trying to do some final tuning before my first ride. The carb i put on it i purchased on eBay. I replaced the jets that were in it as follows:

Pilot: 42
Main: 152
Air Screw: 1.5 turns out
Needle: 3rd position from top
Elevation: 5500

I've messed around w/ the pilot going up and down w/ it. Also messed w/ the airscrew w/ no results. I've had every piece of the carb apart and cleaned extensively w/ carb cleaner and compressed air. Im not too knowlegable on jetting, but usually i can get by. This one is stumping me.

Anyone have any suggestions?

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  • grayracer513

Posted June 06, 2011 - 08:29 AM


Start with the basics:
  • Is the vacuum release plate on the slide installed right way down and free of cracks and chips?
  • Is there an O-ring on the pilot screw?
  • Is the pilot jet really clean?*

* A #42 pilot has a metering orifice of .0165". A film of dry varnish only .001" thick would turn it into a #35. Soak the suspect jet in acetone, toluene, or straight fuel injector cleaner over night, then check the orifice with something recommended here:


Check the passageways leading from the mouth of the carb back through the two air jets.

Be sure your float level is high enough.

Occasionally, I see where someone has broken off a part of the well that the pilot jet lives in, and this can make it impossible for the jet to lift fuel out of the bowl.

  • smdonald

Posted June 06, 2011 - 07:04 PM


vacuum release plate was on backwards. corrected that and made it a "little better". went to a 40 pilot as that was all i had laying around since the yami shop is closed mondays... made it a little better. looked at timing tried retarding the exhaust cam 1 tooth as it looked like it could have been a little off....not sure if that helped or not. pilot also did have the o-ring in good condition.

overall i think all these small adjustments made a noticable difference. it still will not idle at all w/o the choke on. from the jetting charts i could find and the posts i read, the best suited pilot for my riding conditions is a 39. does that seem right? and will going from a 40 to a 39 really give me the large adjustment im looking for? tomorrow I will pick up a 39 and 3 or 4 sizes smaller. also i have the mix screw all the way in. backing it out makes no difference.

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