High-Altitude Jetting for Dummies

A reference for the PWK carbs...
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Wudscrasher
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High-Altitude Jetting for Dummies

Post by Wudscrasher »

I just got the following e-mail from one of our Club members (http://www.nctr.net) who has owned KDXs for over 20 years, and has a dearth of data on jetting specs for almost every model. What I've posted below is specifically for going from high-altitude (5,000' to 12,000') down to mid-altitude (~2,000' and up) for the +/- yr. 2000 KDX 220. I typically ride from 5-12k, but am going to EBF Utah near Lake Powell at the end of April, where the elevations are around ~2500', but only vary about -500'/+1,000. The first part, in italics, is my post to him with my bike and riding specs, his answer is in bold

From: CJ Mueller
>
> Hi, Dan! If it's not too much of a bother, could you give me some advise on
> where to go with the jetting when Jeff & I head to S. Utah next month?
>
> Specs are as follows:
>
> 2000 KDX220, no carb or head mods
> VForce3 Reed cage
> FMF Gnarly "Torque" (-35) Pipe
> FMF TurbineCoreIIQ Silencer
> Airbox snorkel removed, several holes drilled in top of airbox
> I run 13/51 gearing, so I'm in the power relatively quickly, and can usually
> ride around
> Willow Creek in 3rd & 4th, occasionally dropping to 2nd on the steep loose
> stuff. I work
> the motor pretty hard, but when it's spot-on, it does just great! And, I can
> average ~40 mpg or about 120 on a tank.
>
> Current jetting, which is spot-on riding on the Upper Elevations of Willow
> Creek, and even Pawnee (hardly any spooge out of the exhaust):
>
> R1174/2AF needle (your recommendation)
> Clip in 2nd groove from the top
> (I also have the stock R1173 needle. I don't know what the difference between
> the 2 is)
>
> Main: 142
> Pilot: 42
> Airscrew: 1½ turns out
>
> Jeff returned from the Swell and said his bike was running rich, and he even
> fouled a plug (KTM300), after his was running spot-on here
> in Colorado. If that's generally the case, I'll get a few jets, pilot & main,
> that are a little leaner (I think?), or the other way around? I have a 145
> main jet, but don't know if that's leaner or richer. Jeff & I are planning on
> doing a few jetting runs the afternoon we arrive, so if you can get me
> close, that would be awesome!


C.J., sorry I didn't check this account last week. I hope I'm in time to help out.

Do you happen to know the elevation of the Swell? I can find it easily enough but I figured it's the sort of thing a GIS guy would have memorized...

The reason I'm asking is your reference to Jeff's KTM jetting experience. A couple of thousand feet of elevation change shouldn't make that much of a difference on your bikes. I usually jet mine ok at 5K, and don't change all the way up to 12k+. So if he's too rich there, the elevation is a lot higher than here -- which isn't what I thought.

Anyway, my KDX notes show you're right on what we ran for 5K+, approx 50F and up. I wouldn't change anything unless you're having problems. If you are, here are the general rules for jetting:

Main and pilot jets: Main affects wide open throttle (3/4+ open). Pilot affects the beginning throttle just off idle. The # indicates the inner diameter of the jet. Therefore, the bigger the number, the bigger the hole and the *richer* the jet. To go leaner, go to a smaller #.
Needle: The needle affects about 1/4 - 3/4 throttle. The # indicates the outer diameter of the needle. The operational part of the needle fits into a circular orifice in the carb, so a larger number = a larger diameter needle which is a *leaner* needle.
Needle position: You shouldn't need to change the needle itself -- usually just the position will do. That's because the needle is tapered -- narrower on the bottom than the top, but not by much. If you put the clip into a groove farther from the top, you're raising the needle so that a narrower portion is in the orifice. This is richening the mixture. To go leaner, do the opposite -- put the clip in a groove closer to the top of the needle.
Air screw: Affects the throttle just off idle. Screwed all the way in allows no air flow, which means richer. More turns out = more air = leaner.

Good luck with it. I think you'll be fine as-is.

Dan


Hope this helps some of you when you come visit to ride !!!

--C.J.
2000 KDX220R

"Big air? I say keep it in the dirt - that's where the fun is. You want big air, kid? Pull my finger." -- Smooth Johnson, Master of the Berm
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