YZ400F Backfire

2 replies to this topic
  • Hansinator

Posted May 15, 2013 - 01:26 PM


Ive had a 99' Yamaha YZ400F for about 3 years now, and its an awesome bike! bone stock and has been running well for years.
i took it out a couple times over the winter and it was running badly. i assumed this was because it is not fuel injected and jetted for warm weather. i took it out a few times in warmer weather and it still runs badly. the throttle response is not very snappy like it used to be, and it backfires a lot during deceleration. it is also much louder and sounds like crackling instead of the deep sound we have all come to love. i took out the plug (which looked fine), cleaned and gapped it correctly (.029), and i took out the carburetor yesterday, cleaned it as best i could (it also looked fine when i took it out) and put it back in. i started it today and it still runs badly. i thought this was a jetting issue, but could it be that the valves need to be adjusted? it seems like its running hot to me, possibly lean but im not sure. any advice would be helpful!

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  • grayracer513

Posted May 15, 2013 - 02:48 PM



  • rdefonce

Posted May 16, 2013 - 05:24 PM


As GrayRacer alluded to, it's either jetting or air leak.
I had '99 YZ400F, I lived with the popping for a couple years, then got tired of it.
One of two steps richer pilot jet mostly fixed my 400F backfiring.
Also helped to raise the needle one notch for anything other than summer riding conditions.

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