back firing?

12 replies to this topic
  • badburn

Posted April 28, 2009 - 09:12 AM


my new yz450f back fires all the time when i let off the throttle is this normal?

  • RMSpeed6

Posted April 28, 2009 - 10:15 AM


Not normal, get it jetted properly

  • YFZ450_725

Posted April 28, 2009 - 12:25 PM


too rich...head over to the jetting section.

  • JB MCS

Posted April 28, 2009 - 12:35 PM


too rich...head over to the jetting section.

too lean!?

i thought they backfired only when lean. popping off decel...?

clean the carb, and get some jets

  • RMSpeed6

Posted April 28, 2009 - 12:46 PM


^^^ Agreed, it's a lean issue.

  • YFZ450_725

Posted April 28, 2009 - 02:05 PM


Oh, guess I over looked the "decel" part....good catch guys!

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  • grayracer513

Posted April 28, 2009 - 03:35 PM


OK, now that that's over:

Read: http://www.thumperta...599#post3710599

  • rdrata

Posted April 28, 2009 - 08:24 PM


Yeah, but there was some learning going on there...

  • grayracer513

Posted April 28, 2009 - 08:45 PM


Yeah, but there was some learning going on there...

...a fact that seems to have escaped my notice. :banghead:

  • rekros33

Posted April 29, 2009 - 03:30 PM


his bike is an 09.. would it have anything do to with their "new and improved" restricted exhaust system thats on the stock 09 yz450F SE?

why would a brand new bike, with 0 mods done to it, already have jetting issues? the bike has around 10 hours on it.

we ride at 3000-5000 feet, usually in about 5-25 degrees Celsius weather.

would a JD jet kit solve this issue?

or just one size up (from the stock 170 (I think) to a 175?) in the main jet if its running too lean or what?


  • grayracer513

Posted April 29, 2009 - 03:47 PM


It's like that because:

  • Yamaha tends to jet the pilot circuit on the lean side
  • A properly jetted YZ450 should backfire on decel a little, some of the time

The main jet has little or nothing to do with it, and at that altitude, the stock pilot may even work. Start by trying to adjust the pilot fuel mixture screw:


If you believe the pilot jet is still to small, confirm that with this procedure:


Posted April 29, 2009 - 07:57 PM


I just installed the jd kit on my 08 yz450f. I ran a 160 main jet all last season but it was way off once I upgraded my exhaust. The JD kit suggested a 170 for my bike, so thats what I put in it and it does exactly as gray says. Once in a while on decel i'll notice a little backfire. But its not every time I let off the gas. It was also a little cold out today, i expect it to run even better once it gets nicer out. But I will definitely recommend the JD kit to anyone, makes extremely simple for anyone who hasn't done it before (me).

  • rekros33

Posted April 29, 2009 - 08:49 PM


09 yz450f motor is the exact same as 08s motor

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