07 450 exc Carb Mixture Control Screw



New to this forum, thanks for the advice in advance.  Just bought an 07 450 exc and have a lean carb condition right after the throttle is cracked.  My question is what exactly does the mixture control screw (MCS) control, the air flow or the fuel flow?  Also, does anyone know the effective range of adjustment before I need to change the pilot jet?

If the idle mix screw is between the slide and the head it's a fuel screw (in is leaner), if between the slide and the air filter it's an air screw (in is richer). 4st usually fuel & 2st usually air.


In your case it is a fuel screw and the useful range of adjustment is 3/4 to about 2 1/2 turns out from lightly seated. If not between 1-2 turns then I would be looking ant the next pilot jet. The further out the more chance of it vibrating out and if you ever do remove the screw be sure of capturing and the order of the parts.



Edited by BDM

Thanks, good advise about the screw position.

From my experence the mixture screw has little real effect on throttle response. It is, after all, the "Idle mixture screw".  I fought a just off idle bog for way too long. I think mine was too rich off-idle as the problem got much worse as you go up in altitude.


Short story... it was a needle issue. The bike came with the JD blue needle which experts say has an off-idle issue at any altitude on the RFS and is real bad at high altitued. Instead of buying a whole new JD kit to get the red (high altitude) needle, I went with the KTM ODBTR needle. The bike is 100% better. Supper smooth at low trottle setting on tight trails... Love it!


I run a 42 pilot and never ride below 4,000ft. at 8,000ft I turn in the mixture screw a full turn and this improves the idle at altitued. (screw in = lean it out at high altitude).

Edited by DrKayak

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