2008 yz450f jetting/bottom end problems HELP!!

7 replies to this topic
  • dylan01

Posted August 13, 2011 - 08:23 AM


Hey guys,

I bought the yz450f at the beginning of last year and have had this bottom end problem since I bought it.
The previous owner put in hot cams and changed the jetting
Stock jetting is 45/162 and he changed it to 48/168
When I first got the bike the problem was really bad but I fixed it a lot by moving the clip of the needle to the very top, moving the needle all the way down.
Next I tore the bike apart and cleaned everything from the carb and changed the main jet to 165
The bike was actually running good for 1 day, but the next day the problem returned
After that I took the carb apart again and put in a 42 pilot jet
Bike still has the bottom end bog and now it backfires on deceleration.

I really need help with this. The mid and top end power of this bike is amazing I just need to fix the speed out of the corners.
It's really annoying.

Thanks in advance for any advice!

  • shrubitup

Posted August 13, 2011 - 08:31 AM


Try a richer leak jet.

  • grayracer513

Posted August 13, 2011 - 10:22 AM


Try a richer leak jet.


Backfire on decel has nothing to do with the main circuit (main and needle) or the accelerator pump. It's usually caused by a pilot that is too small or that has a film of varnish dried into it, making it smaller than it's supposed to be. This will cause poor throttle response and stumbling also. Read:


A bike that runs well otherwise, but backfires excessively likely has an air leak in the exhaust system.

  • shrubitup

Posted August 13, 2011 - 11:12 AM


What was wrong with the stock 45?

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  • grayracer513

Posted August 13, 2011 - 02:49 PM


Good point; a 42 is too small unless you're at pretty high altitude.

  • YZPaGuy

Posted August 13, 2011 - 03:31 PM


I have an 08 and I run a 165 main and a 48 pilot. Had decel popping with the 45 stock. My 08 had a 45 pilot and 160 main stock. Search thos forum. Everything you need to do is on here.

  • dylan01

Posted August 14, 2011 - 09:53 AM


Thanks guys
I meant I put in the stock 45 pilot
And that causes the popping on decel
I'm going to put the 48 back in and check the colour of the spark plug
See if that works


  • ttr250dude

Posted August 18, 2011 - 10:16 PM


48 pilot, 165 main, 1 5/8-2 1/4 turns out on the fuel screw. If you have a problem with low end bogging you should look at your fuel screw. If your screw is turned more than 2 1/2 turns out, your pilot is too small. If your screw is less than 1 1/2 turns out, your pilot is too big.

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