KTM 250 SX Carb Swap

kylant wrote:

^^ thanks, enjoy

rocketsocks26 wrote:

Im selling a 38mm carb off my 15 300sx it has 3 hrs on it since new, $350 with the necj needle mxboy2661@yahoo.com

digger wrote:

It looks to me like you have a 38mm PWK off an early big bore(380-500) bike. The N85C needle is an old school duel taper needle...

It looks to me like you have a 38mm PWK off an early big bore(380-500) bike.

The N85C needle is an old school duel taper needle that was used in larger cc bikes. It has the same exact taper and profile as a modern YZ/KTM N3xx needle, but with a leaner L-1, which means the clip positions are 1 full clip position leaner than a Yamaha/KTM N3Cx needle. N3Cx in #2 clip = N85x in clip #3.

The part on that needle that is causing most of your problems is the C diameter of the needle. This will affect your low throttle setting(0-1/4). With that said you will never be able to get that needle to run cleanly down low. The diameter is just to rich to be useful for you on your 250, IMO.

The Suzuki needle setup is where I would start out if I were you. Start out with an NECJ needle in #3, 40/42 pilot, as determined by the AS, and start out with a 172/170 main.

digger wrote:

It looks to me like you have a 38mm PWK off an early big bore(380-500) bike. The N85C needle is an old school duel taper needle...

It looks to me like you have a 38mm PWK off an early big bore(380-500) bike.

The N85C needle is an old school duel taper needle that was used in larger cc bikes. It has the same exact taper and profile as a modern YZ/KTM N3xx needle, but with a leaner L-1, which means the clip positions are 1 full clip position leaner than a Yamaha/KTM N3Cx needle. N3Cx in #2 clip = N85x in clip #3.

The part on that needle that is causing most of your problems is the C diameter of the needle. This will affect your low throttle setting(0-1/4). With that said you will never be able to get that needle to run cleanly down low. The diameter is just to rich to be useful for you on your 250, IMO.

The Suzuki needle setup is where I would start out if I were you. Start out with an NECJ needle in #3, 40/42 pilot, as determined by the AS, and start out with a 172/170 main.

YZed250 wrote:

I tried your suggested brass, digger and my bike still loaded up in the first half throttle really badly and could only be cured by switching...

I tried your suggested brass, digger and my bike still loaded up in the first half throttle really badly and could only be cured by switching off the gas, and even then, it would only run decent for the tiny window between running out, and totally starved for fuel.
I never quite figured out why it wouldn't run with the 38 mm carb, but would like to now.
FWIW, I installed my stock 36mm carb back on the bike and it rips.
About the only thing that I did check, but wasn't 100% sure on was the float level. Float level seemed to be in the ballpark. Could that be what's causing it to load up in the lower half of the rev range? Other ideas?
You'll see a previous picture I posted of an epoxied passage on the intake side. I strongly suspect that the guy that installed the wing (previous owner/ tuner) did that to help with flow.

Charper732 wrote:

Your post is a little confusing as you say its loading up and then its starving for fuel. A lot of people confuse too lean with...

Your post is a little confusing as you say its loading up and then its starving for fuel. A lot of people confuse too lean with being too rich and vise versa...on 2 strokes anyway. I'm at 1064ft elevation and I ended up with a 44 pilot as I had the air screw at less than one turn out on the 42 And I dropped the NECJ needle one clip from the middle.
The air screw will make you bog like hell off idle if you don't have it properly set.

Float level is extremely important as it effects all the circuits of a carb. Make sure its on point and just tinker away. These carbs are super easy to work on since everything is there in the open.

digger wrote:

Do yourself a favor and startout with a main jet 2 to 3 sizes larger than what you were using in the 36. The decrease in...

Do yourself a favor and startout with a main jet 2 to 3 sizes larger than what you were using in the 36. The decrease in the velocity of the air that is drawn through the 38mm carb will require a larger main jet. You also may need to increase the pilot jet 1-2 sizes as well to startout with.

kylant wrote:

with this advice, do you suggest doing anything with the needle? i was running a 40p, NECJ #3, 160 main, as 2 prior to swapping to...

with this advice, do you suggest doing anything with the needle?

i was running a 40p, NECJ #3, 160 main, as 2 prior to swapping to the 38, NECJ#2, 158, but the air temps were a good 10* cooler. bike still ran good, but I think it was a little fat on the bottom.

sounds like maybe I should go back to the richer jetting for the 38 carb and see how she runs?!


oh and BTW, I got the carb listed for a 2014 150sx, and it came with a 7 slide instead of the 6.5 posted earlier. KTM must have made a change. I tried to get an earlier model carb, but this was the only one available without a long backorder

digger wrote:

You've got the right carb with the right slide. That NECJ needle is fine to start out with. Ideally you want to start out a little...

You've got the right carb with the right slide.

That NECJ needle is fine to start out with. Ideally you want to start out a little rich and work your way leaner, especially with the main jet. With that said, start out with the Needle in the #3 clip, with the 40 pilot and a 168 main and work your way leaner, if you need to.

kylant wrote:

with this advice, do you suggest doing anything with the needle? i was running a 40p, NECJ #3, 160 main, as 2 prior to swapping to...

with this advice, do you suggest doing anything with the needle?

i was running a 40p, NECJ #3, 160 main, as 2 prior to swapping to the 38, NECJ#2, 158, but the air temps were a good 10* cooler. bike still ran good, but I think it was a little fat on the bottom.

sounds like maybe I should go back to the richer jetting for the 38 carb and see how she runs?!


oh and BTW, I got the carb listed for a 2014 150sx, and it came with a 7 slide instead of the 6.5 posted earlier. KTM must have made a change. I tried to get an earlier model carb, but this was the only one available without a long backorder

digger wrote:

You've got the right carb with the right slide. That NECJ needle is fine to start out with. Ideally you want to start out a little...

You've got the right carb with the right slide.

That NECJ needle is fine to start out with. Ideally you want to start out a little rich and work your way leaner, especially with the main jet. With that said, start out with the Needle in the #3 clip, with the 40 pilot and a 168 main and work your way leaner, if you need to.

Crash153 wrote:

Hey Digger, you seem to be the go to guy on this mod. Im struggling with my set up. I have the 38mm NECJ 3 40...

Hey Digger, you seem to be the go to guy on this mod. Im struggling with my set up. I have the 38mm NECJ 3 40 170 AS 1.5. Seemed ok but a little lazy to get on the pipe, had to really clutch it. I checked the plug and it was a little dark so I move the clip to 2. Sounded good, it was awesome on some real nasty single track, chugged like a tractor, idle was as smooth as Ive ever seen, but when I got on the open again it felt a little lazy to get on the pipe. We started at 5k and rode up to 7k, at that elevation I couldn't test the power too much as it was billy goat territory.
.
im currently watching Jarvis and his machine has kinda the same boggy sound as mine. Maybe im chasing a phantom problem.

It ran pretty good and pulled hard in the mid with the JD Red 3-2 clip depending on the elevation.

But the best this bike had ever run was with my 36mm JD Blue 2 40 168. Freaking super crispy!!! I want to believe this 38 could be better though.

Almost forgot to give you the stats!!!
Im in AZ and ride from 4-6k,
SX Head
SX CDI