Hi guys!
i have a kx 250 '99 and i was thinking to make a jetting modification. right now my jetting specs are the following:
pilot:48
needle: n3yk - fourth position
main: 172
air screw:1-1/2
my bike with this jetting is a little bit rich from 1/4 to 2/4 and a little bit lean from 2/4 to full throttle.

reading some threads on the forum some guys advise a n3wk needle in 3d position with a modification on the power jet from #55 to #58 and others a n3we needle..

any thoughts?
thanks in advance

Hello,

First thing that is important is to understand how the carb operates. idle to 1/4 throtle is generally pilot jet, 1/4-3/4 is going to be needle, and 3/4-WOT is main jet. It sounds like we may need to bump up a main jet and possibly drop the needle down 1 clip. After this I would ride the bike for a good hour and see what you got for plug color and determine if you liked the feel. Generally the bike will not move from super rich to lean like that. What are we using for plugs, what are we mixing our oil/fuel? These are important factors when it comes to running the bike as well.

hello and thanks for your response ktmhiflyer,

you are absolutely right for the reccomendations. today i will start from testing the pilot circuit with a leaner pilot jet (#45). i run 50 to 1 mix ratio and a BR8-something spark plug..
my question above was mainly for the needles.. if someone know how they operate.. (N3WK and N3WE)
i was thinking to change the needle because i readed somewere that the needle clip is better to be placed in the middle positions

osvi, on 02 June 2012 - 02:24 AM, said:

The needle comes with 5 different clip positions so that you can adjust it to your liking. Whether it's clip 1 or clip 5 is irrelevant other than you have no more adjustment in the direction those clip positions favor. As to the needles, the tapers on N3W_ are the same as those on N3Y_ but they are located 1/2 clip position closer to the clip grooves (richer) on N3W_. This means that when installed in the same clip position, N3W_ will run a 1/2 clip richer than N3Y_. So if you have both needles you can fine tune in 1/2 clip increments instead of full clip increments. A clip position change will have a varied affect from @ 0 to 7/8 throttle with the greatest affect occuring  @ 1/4 throttle.
The last letter on these needles dictates it's starting diameter or straight section diameter. A change in starting diameter has no bearing on the diameter of the rest of the needle. So with these needles the starting diameter only has an effect in a small throttle range from @ 0-1/8 throttle. There is a specific diameter for each letter. Starting with A = 2.655mm then increasing .05mm for each letter with some minor exceptions. The letter O isn't used and the letters I and W share the same diameter of 2.735mm. The larger the starting diameter, the leaner the needle will run in the area affected by starting diameter, again @ 0-1/8 throttle.
So using the needles you posted...
N3WE- E has a starting diameter of 2.695mm
N3WK and N3YK- K has a starting diameter of 2.755mm

N3WE-clip#4 will run richer 0-1/8 throttle than N3WK-clip #4.
N3WK clip #4 will run a 1/2 clip richer than N3YK clip #4. And N3WK-clip#3 will run a 1/2 clip leaner than N3YK-clip#4.
Don't forget that a clip position change has an effect over a wide throttle range (0-7/8), so it can have some effect in the same area as a the starting diameter.

Seems odd to me that your running a 172 main and it feels lean. Wasn't the stock main for 99' a 158?

thanks a lot hallsy,

your explanation is great. now everything seems more clear!
i did the modification on the pilot mentioned above and the throttle response was a little better to full throttle to. so maybe you are right about the main jet, maybe is to rich..
the fact is that i renicasiled the cylinder and i had to make it 1mm larger so the stock specs doesnt work so good in my bike.
now about the needle if i have to try something else i might go for the N3WK as the pilot circuit works fine right now

thanks again