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

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:

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


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.....

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!

Thank you very much!!! :applause:

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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