KDXRIDER.NET

I've got a KDX220 motor in a '99 CR 250 frame with a modified '96 cr 250 exhaust on it. To get the exhaust to fit, I had to add ~2" of straight just outside the cylinder.

The motor in stock form ran very well. I ran jetting of 142 main, 45 pilot and the stock needle (1173). It had an occasional stumble right of idle under load, but nothing I couldn't ride around.

I swapped over my RBD carb and head from the real KDX, so my jetting is now 150 main, 38 pilot, CEK needle with the clip in the 3 position.

With the mixture screw 7/8 out, the stumble under load off idle was more pronounced. With the mixture screw 1-3/8 out it is less but still there. In fact, I can make it happen if I want, so it is pretty predictable. The motor has more power everywhere, especially on top.

My question is, which way do you all think I should go with the jetting? I will gladly sacrifice some of that top end and some of that hit for more and cleaner bottom, if I have to. I realize the pipe is a complete unknown.

Thanks for any help!

May need a needle with a slightly leaner 'straight' section, or more taper. What needle is in there now? May need a slightly leaner slide, but start w/ the needle for sure. All to do w/ jetting for the pipe. Probably a lot easier to work w/ the carb then re-do the pipe. I will add that you went the right way w/ the pipe if you had to do so....A shorter header will increase top-end power, and will also run hotter. Basically, you installed a 2" 'torque ring,' and I must say I do find it interesting that you have great top-end at this point. Guess that it's due to the fact that it was a 'rev' pipe for a 250 to start with. Musical pipes is fun! Never know what's going to happen.

I've got the CEK in there now. Would the stock(1173) needle be going in the right direction?

The pipe geometry does not look a whole lot different than the stock KDX pipe. The straight inlet section on the CR pipe was pretty short.

Take a look at this chart. It is a necessity for picking needles:

http://www.sudco.com/keihin_jn3.html#pj

You're probably going to need something slightly unorthodox for what you see on here all the time. I'd say grab a few, and see what kind of results you get. I'd start with maybe a CEN, BEL, BEN

Thanks for the chart, Fuzzy. Very handy. Any Idea where the stock 1173 needle falls in there?

Last night I changed to a 40 pilot and the stock needle. Off-idle response is worse.

The way I'm defining performance is to idle along in 3rd and snap the throttle to 1/4 or 1/2 or even full. I would of course like it to pull clean

smaller pilot first 35, make one change at a time. 0-1/4 is pilot circuit, 1/4-3/4 is needle, 3/4- 1 is main jet. they do over lap some what so a change in one can effect the other.

the stumble is a rich condition off idle ? or a lean condition?

sputter gurgle =rich

bog = lean

by adding to the exhaust you change the dynamics of the pipe

so abnormal thinking and jetting will apply to an extent.

Abnormal thinking is O.K. with me. Just ask my wife.....

Rich or lean, I am not sure. I think lean (BuuuWAaah?)

I bet the 1173 needle is leaner than the CEK?

I'll change back to the CEK needle, clip in the middle.

I've changed back to the CEK, clip at three from top

Jetting CEK-3, 40, 150

All is right with the world

The stumble is gone, the midrange pull is clean. Suprising what a difference that needle makes. I think I'll pick up a DEK, just for the heck of it..

Now to check the main.

Sweet!