Jetting headache 2009yz450f

7 replies to this topic
  • jesse2982

Posted May 02, 2012 - 08:45 PM


Hey guys i have a 2009 yz450f with an fmf 4.1 and megabomb header. I have had issues with the jetting on this bike since i changed the exhaust, i just put in a 165main jet and raised the needle 1 clip position, i had my fuel screw set at 2 3/8 turns. The bike pops alot of decel still but throttle response and overall power is better. But the bike isnt consistent and doesnt run great. Im not sure where to go from here to get it running great. I live in socal ride mostly milestone and lake elsinore. Any help would be much appreciated.

  • RiderX

Posted May 02, 2012 - 11:09 PM


What pilot are you running?

  • tpars121

Posted May 03, 2012 - 11:05 AM


I have an 08 with the same pipe and header and I called FMF and they gave me their recommended jetting over the phone. I think it was a 165 main, 50 pilot and 1 1/2 turns

  • grayracer513

Posted May 03, 2012 - 12:13 PM


50 pilot ...

Sounds way too big.


The decel popping is likely to be caused more by air leaks in the exhaust system. Common with FMF's.

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  • tpars121

Posted May 04, 2012 - 04:27 AM


Well, I had some doubts so I checked my notes and that is whats in there and it may be a little on the big side but it's what FMF recommended. I guess my point in the earlier thread is to call FMF and see what they recommend as a starting position since the OP seems to be guessing at this point.

  • Steel Panther

Posted May 06, 2012 - 01:02 AM


I have the same bike and exhaust, I run a 165, needle 4th position and fuel screw somewhere... Like gray said that exhaust needs to be lined up perfect everywhere or it will leak and cause some popping.

  • AliGee

Posted May 06, 2012 - 03:47 PM


Get a 2010+, that'll solve your problem! Sorry I just had to... :banghead:
I will never, NEVER miss all my jetting headaches...

  • rcdude33

Posted May 07, 2012 - 06:22 PM


Here you go man:


I absolutley love my jd jetting, it takes all guess work out. My bike has crisp throttle response and it pops a little on decel but it doesnt sound like a gun fire going off haha.

But keep in mind that i had some other mods that were done to my carb too. And like gray said if the exhaust has a leak the bike will pop. I bent my header and after i did my bike non stop popped all the time.

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