Jetting??? How do I know if I need Jetting done???

6 replies to this topic
  • JoeJoe03

Posted May 29, 2006 - 11:32 AM


Hey Thumper Pros,

I have an '03 YZ450F and live in good ole Houston Texas. I have added a couple mods to my bike and now I am wondering ....

How do I tell if I need Jetting????

Is there any simple test I can do to tell if I need a smaller or larger jet? I want to reach optimum performance! I have an air mixture screw that I mess with sometimes but I dont know exactly what I am listening for...

What are the tests or signs you look for that can tell if jetting is needed?? Then how can i tell when my jetting is right on????

Thanks a bunch :applause:

  • JoeJoe03

Posted May 30, 2006 - 05:02 AM


Anyone??? Or can you tell me where to find this info?


  • Ga426owner

Posted May 30, 2006 - 08:40 AM


Chances are if the jetting is stock spec it needs some type of rejetting adjustment. For Ga 900-1900ft on my 03 I started with adjusting the clip up 1 or down 1 position. I only run VP U4 and this caused a leaner running condition. I did a few changes to the breathing and carb - powernow, Twinair powerflow, quickshot that changed my bikes jetting also - I opted for a JD Jetting kit and once I got the jetting spec right - it changed the power of my 03 dramatically....much more hit more linear power through the RPM range and perfect throttle response.

But you have to figure out what the symtoms are that would cause you to rejet - A aftermkt pipe my change it, or any of the things I mentioned. And of course what type of rider you are to. Is the bike sluggish at any rpm range? does it pop like hell? Is the sparkplug rich? etc.....

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  • JoeJoe03

Posted May 30, 2006 - 10:06 AM


My bike has the following:

Factory 4 with a power bomb header
Power Now
Power Now Plus
Airbox breathers
Twin Air filter

I dont get bog but some popping on the deceleration but not much...I run 93 octane from the pump only. I ride MX mostly. I would like more throttle response (low end!!!)

I read somewhere that the amount of turns of the "fuel mixture screw" would indicate if I need smaller or larger jets...Is this true and how is it done??

What steps would you recommend I take to get optimum performance??

THANK YOU VERY MUCH!!!!!!!!!!! I appreciate ya'lls time!

  • Ga426owner

Posted May 30, 2006 - 10:20 AM


Check these out - should be helpful

lots of jetting help

Fuel screw instruction:

For the best performance jetting:

Oh and I though "Ya'll" was a Ga/Al saying.... :applause:

  • JoeJoe03

Posted May 30, 2006 - 04:39 PM


Thank you very much!!! :applause:

  • kawirider

Posted May 30, 2006 - 06:39 PM


My bike has the following:

Factory 4 with a power bomb header
Power Now
Power Now Plus

You say it only pops a little bit on decel right? In that case you are fine. A little bit of popping is normal. But as far as you wanting better throttle response, why do you need it? the '03 is very snappy even without an aftermarket exhaust systema and carb mods. But if you just have to have better throttle response I would recomend a JD jet kit.

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