Another yz426 backfiring

11 replies to this topic
  • Biggie9367

Posted April 20, 2016 - 09:07 AM


Ok so my bike is backfiring on accel and decel. Also, it is hard to start, no idle adjustment and fuel mixture screw quarter turn out to start. When I bought bike it would only run with choke on. Changed jets with no luck so cleaned and bought another parts carb to create one good carb. Put all stock mains and pilots in after rebuild. Put carb on but now only starts with choke on, and if I keep it revved it will run with the choke off. Did the mixture screw check with only a noticeable bog down about 3 turns out. Took valve cover off to confirm timing chain and left can gear was off a tooth. Adjusted but now not sure if I should try and start or adjusts jets too while apart? Any suggestions?

  • kNewc

Posted April 20, 2016 - 09:20 AM


Sounds like valves to me. All the symptoms - backfiring and hard to start.  Also, if you had the timing off you could have bent a valve and possibly have a big fix on your hands. I only know that because of my RMZ 450 I have now lol

Edited by kNewc, April 20, 2016 - 09:21 AM.

  • grayracer513

Posted April 20, 2016 - 09:53 AM



Did the mixture screw check with only a noticeable bog down about 3 turns out.


That indicates your pilot jet is too small.  You could try it again now that you have the timing back where it belongs, but 3 turns out means either too small a jet or you still have something obstructing the pilot circuit.

  • Biggie9367

Posted April 21, 2016 - 05:43 AM


Ok I will try the suggestions, also what does the adjustment screw do under the throttle cable wheel on the carb?

  • grayracer513

Posted April 21, 2016 - 05:56 AM


The screw that's attached by a cable to a black plastic knob is the idle speed. 


The small screw in the linkage below the throttle cable wheel is for accelerator pump timing.

  • Biggie9367

Posted April 21, 2016 - 07:02 AM



The screw against the black rocker does pump timing which will need adjusted for what scenario?

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  • grayracer513

Posted April 21, 2016 - 08:36 AM


Almost never needs adjusting.  The pump is supposed to be timed so that the squirt is as early as possible WITHOUT hitting the slide as it opens.




  • DinoY

Posted April 21, 2016 - 09:35 AM


Did you check for cracks on the carburetor manifold?
If air is leaking you wont be able to tune the carb

  • Biggie9367

Posted April 21, 2016 - 11:39 AM


No cracks but also no hoses, as when I rebuilt it, the guy forgot to give them back. May be a problem but from others I've talked to it should have little bearing on the backfiring

  • Wanny14c

Posted May 01, 2016 - 05:01 PM


Just curious if you figured out the problem? Reason for asking is I rebuilt my carb and took the top off to get at the slide and I took a screw out which slid the whole assembly over and I was not able to set the idle. When I finally figured it out it started first kick.

  • Wanny14c

Posted May 01, 2016 - 05:04 PM


Never mind.

  • Biggie9367

Posted May 02, 2016 - 06:10 AM


So still no change, adjusted timing one mark. Have not had chance to check cable clearance yet but did notice the pilot jet was what looked to be clogged and cleaned it out. Will throw in tonight and see if anything has helped

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