jetting query....... help please

hi all

i have just fitted an aftermarket fuel screw and would like to know which way to turn to make more rich?

i have also fitted 168 main 48 pilot and adjustable needle with circlip on center groove. she starts fine but cuts out when throttle opens to quick. mid range is fine but top end is spluttery/wont pick up.

the spark plug is white so i know the mixture is lean and when revved up the airbox seems to make a sucking type of sound.

the fuel screw is currently 1 1/2 turns out.

i will try moving circlip down one groove to make rich and adjust fuel screw to make rich, but if this does not sort out lean mixture shall i fit a 170 main.

thanks for any suggestions

btw is 06 wr450 0 to 4000 alt and cool temp

I have the same bike and did all the free mods. I have the same jetting set up as you mention and my riding alt range is the same as yours. Mine was running lean so I took it to the local shop. They put it on their dyno and had to adjust the air/ful screw out 2 3/4 turns. Runs great, no hesitation and smooth through the throttle range. My needle clip is in the 4th position which is the suggested spot per the AIS Removal kit. Try this before bumpping up your main to 170.

Unlike a 2 stroke with an air screw that meters air, a 4 stroke has a fuel screw that meters fuel. Also unlike a 2T, you richen the mixture by turning the fuel screw out.

...she starts fine but cuts out when throttle opens to quick.
This is probably due to the AP circuit. You'll need to tune your leak jet and the pump timing. Check the sticky's in the 250F forum for the FAQ.
...mid range is fine but top end is spluttery/wont pick up.

Not sure; the symptoms sound too rich, but I would guess that you need to go richer, still, only because I'm guessing it's cold and wet over there (however, the cold requires richer jetting while the wet requires leaner jetting). Fortunately, the mj is the easiest to change. try a 165 and then a 170 and see which is better.:applause:

The top end sputter may be caused by the TPS sensor deal people keep talking about in these posts. Follow Eddies pilot jet check and make sure the pilot is right first. Once you do that, unhook the TPS (Throttle Position Sensor.) and see what happens. Also check for vacuum leaks.

Good Luck !


Try going to There's a great "How To" section on tuning the fuel mixture with the screw. It's really simple and takes just a minute or two.:applause:

Hi, just a thought about the white plug and sucking sound. Not evaluating with my own hands, eyes and ears - I may be way off but, something on your intake may be loose and you are getting unfiltered air right into your carb. Check to see if everything is seated properly and tightened properly. Just a thought. Jason

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