yz 400f carburetor problems

7 replies to this topic
  • glamisemt

Posted June 30, 2016 - 07:40 AM


So I bought a used yz runs fine only with the choke on. Will not idle without it. I rebuilt the carb myself put in New pilot jet . old one was plugged put her back together and now I'm having the same issue again. Any ideas before I tear down the bike again to get to the carb?

  • Krannie McKranface

Posted June 30, 2016 - 03:20 PM


The passage above the pilot jet has a 90 degree bend in it, and it corrodes/clogs.


You can use #15 or larger fishing line and spray cleaner up the passage.


If you use 'real' carb cleaner, you will need to bind your thottle to wide open so you don't get any cleaner on the slide seal.


Also, the choke plunger can fail, so the choke is always on...

  • glamisemt

Posted June 30, 2016 - 04:14 PM


Got it thanks

  • glamisemt

Posted June 30, 2016 - 04:51 PM


Krannie can I access that by breaking down the carb further or is it an area I cannot reach

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  • grayracer513

Posted July 01, 2016 - 06:58 AM


It's accessible by removing the pilot jet and working up from there, and by working down from the orifice in the carb throat on the engine side of the slide. 


DO NOT separate the upper and lower  carb body unless you first have in hand a replacement gasket.  That gasket is not serviced by Yamaha or Keihin, and is available only from JD Jetting, as far as I know. 


Carb cleaners such as Berryman's B12, which contain mostly toluene and methyl alcohol, will not permanently damage the slide seal or any other carburetor component.  If the seal is left in place, it will be fine.  If removed, and it appears swollen and distorted, allowing it to dry for 24 hours will usually return it to its normal size and shape.

  • glamisemt

Posted July 01, 2016 - 08:52 AM


Got it thanks for the info. That pretty much answers my question. Think I'm going to have to take it in and have it done since I don't have that gasket. I'm guessing there's a buildup somewhere in there and will need to have the carb cleaned by a shop

  • glamisemt

Posted July 01, 2016 - 09:00 AM


Fuel screw tool I made

Attached Thumbnails

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  • glamisemt

Posted July 03, 2016 - 11:10 AM


Alright got it all apart again. Pilot jet was not plugged. I bought a bigger pilot jet one size up and a visible fuel screw. Could this be just an adjustment issue? Going to try 2 turns out

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