Yz426f Wont idle

6 replies to this topic
  • Mr426f

Posted July 21, 2016 - 01:11 PM


Read other post but cant get it solved, checked pilot, its cleaned but still wont idle, played with fuel air screw, and now once i push in choke it back fires , instead of just dying now? Any ideas

  • Krannie McKranface

Posted July 21, 2016 - 05:18 PM


Read other post but cant get it solved, checked pilot, its cleaned but still wont idle, played with fuel air screw, and now once i push in choke it back fires , instead of just dying now? Any ideas


Yes, it's time to do a complete carb service, including replacing the old worn parts




You must ream the passage above the pilot jet (tight bend) replace the needle jet (wears out) and slide plate seal, and put in a BRAND NEW pilot jet.

Service the hot start and choke systems.

  • grayracer513

Posted July 23, 2016 - 10:08 AM


There simply is almost never a reason to discard a pilot jet if it's cleaned correctly. 


Read: http://www.thumperta...o/#entry6879695


If you don't have access to something such as a set of small number drills, replacing the jet is another approach.  Not terribly expensive, but again, rarely necessary. 


The carb passageways are another matter.  Take a look at the picture attached. #6 is the pilot jet, 7 is the fuel screw, and 3 is the pilot air jet.  Note the passageways connecting these components and the fact that there are two discharge ports (red arrows).  The one toward the engine is what the bike mainly idles on, while the one forward and under the edge of the slide helps transition the engine off of the idle circuit and onto the main circuit as the throttle is opened. A pick made of very fine wire, or a piece of light mono filament fishing line can be used to check that these are open, and the fishing line can be used to snake out the passageways.


The older Flat FCR as used on the 426 is also prone to gumming up the passageways between the upper and lower body halves.  The factory, neither Yamaha nor Keihin, don't offer the special formed gasket that goes there, and the original one is usually unusable if disassembled.  DO NOT separate the two halves unless you have a gasket in hand first.  JD Jetting sells them.  Send them the number off of the side of your carb and order one befoere you take that step.

Attached Thumbnails

  • carb.png

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  • Krannie McKranface

Posted July 23, 2016 - 11:43 AM


The corrision on the inside of the pilot jet will not be dissolved with spray carb cleaner.

You have to clean it ultrasonically.

It's easier just to replace it.

  • grayracer513

Posted July 24, 2016 - 06:32 AM


90% of the time, it's not corrosion that blocks the pilot jet, it's a film of dry varnish deposits from evaporated fuel.  You are correct that carb cleaners won't usually remove it, but that just means another approach is required. Pushing the correct size number drill through the orifice (not twisting or drilling per se, just pushing it straight through) takes care of the problem in 1 second.  I end up doing this about once a year to my own, as it sits during the three hottest months of the year most of the time.  It's easy and it works.

  • Mr426f

Posted July 25, 2016 - 03:55 PM


Ok ill check that but do the pilot jet suppose to be fully seated?

  • grayracer513

Posted July 26, 2016 - 06:51 AM


The jet gets seated, the screw is gently seated, then backed out 1 3/4 turn as a baseline setting.

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