2017 KTM 250 exc Keihin carb jetting

Related:
Create New Tag

1/22/2020 6:16 PM

I thought I would post this to help any one struggling to getting the jetting right on there KTM 250. I swapped out the original Mikuni out of frustration and went to a Keihin PWK 38 because I consider this carb better suited to the bike. I bought a JD jetting kit , which in hindsight was probably caused more confusion. With JD jetting in, the bike had a pipebang / de acceleration chug making it unsettled into corners and spooge like no other. I was committed to get to the bottom of this and set about learning more on this forum and many others. After reading and trying to consume as much info as I can I came up with the following: JD jetting red needle on 2 nd clip ( will be swapped out for N3CH needle when it arrives), 38 pilot,165 main, a/s at 3 turns out. The bike revs free, no pipebang and no spooge. I ride at 0-3000ft in Queensland
, Australia. Not a believer in JD jetting anymore or their advice given when contacted. I hope this helps someone chasing the right settings for this bike and carb setup.
Cheers

|

1/23/2020 6:18 AM

Whats amazing to me is that people will put all that work into the Keihin, but not into the Mikuni and when both are sorted out run the same and would be less expensive to mod a slide and buy a few mains, pilots and a needle.

|

2018 KTM 250sx
Instagram CamaroAJ

1/23/2020 8:47 AM

AJ, I respect your opinion on the Mik, in fact your jetting specs did work about the best I got it to work. I'm not sure how consistent your weather in Texas is and maybe that's why I've had a different experience than you. Here in New England we get pretty wide swings in temp and humidity. While I could have the bike running perfectly it could then be a blubbering mess in the matter of a few hours. When we finally went to the Keihin this problem went away. I even went back to the old NECJ needle I used to love and it's still the ticket. Steve, I know the JD needles require different mains so maybe that's why you're ok with the 165m, that sounds pretty lean to me though so be careful if you do change away from the JD.

|

1/23/2020 10:16 AM

Well AJ, amaze yourself no more. In my opinion, the Mikuni is a mismatch with this bike. It was an after thought for KTM who wanted this to be the model to have the TPI but it wasn’t ready to go into production and Mikuni supplied a carb to fit the hole. Keihin has been the carb of choice for KTM on their previous models with no problems. Here in Queensland Australia we get hot temps with humidity and the Mikuni was just too sensitive. I on sold the the Mikuni and I bought the Keihin PWK 38, which in my opinion is much better suited carb. Once jetted correctly, which doesn’t include JD ‘s suggestion, once again , in my opinion, it runs sweet. The pipebang/ de acceleration chug has gone and the bike runs sweet..... in my opinion of course

|

1/23/2020 10:42 AM

Hellion, cheers mate, I do intend on going to a 170 main when swapping the needle to the N3CH needle.

|

1/23/2020 10:53 AM

hellion wrote:

AJ, I respect your opinion on the Mik, in fact your jetting specs did work about the best I got it to work. I'm not sure how consistent your weather in Texas is and maybe that's why I've had a different experience than you. Here in New England we get pretty wide swings in temp and humidity. While I could have the bike running perfectly it could then be a blubbering mess in the matter of a few hours. When we finally went to the Keihin this problem went away. I even went back to the old NECJ needle I used to love and it's still the ticket. Steve, I know the JD needles require different mains so maybe that's why you're ok with the 165m, that sounds pretty lean to me though so be careful if you do change away from the JD.

There was a post not to long ago by a mag or something that said once both these carbs are correctly jetted that neither are that temperamental with changes in elevation or temp. They even said the Keihin is very temperamental if not jetted right. I'll have to try and find it later.

I run pretty much the same jetting all year from high 30's (this needs a bigger pilot) to 100+ and almost never touch the carb. Every once in a while I'll check the air screw. I've run in low humidity and high without changing a thing. I've been in high humidity with fog in the morning to hot and dry in the afternoon and still didn't need a change.

From what I've been gathering people that are having problems still after changing to my jetting are only doing part of the changes. Like they will cut the slide, but run the main lean and the pilot rich and they say its ok. Or they will run the slide and the jetting, but not the needle and still have a problem. I've also had them run my full specs, but change the power valve and complain it doesn't pull right or how they want it to. Sometimes they will run the full specs. and try and run 32:1 premix with it. It all has to go together to work right. What really needs to happen is for someone else to ride my bike and then post up what their thoughts are on it. Sadly everyone that has ridden it they don't post here on vital lol.

|

2018 KTM 250sx
Instagram CamaroAJ

1/23/2020 11:31 AM

The Keihin vs. Mikuni carb debate goes on and on...... Over the years I have swapped more than a few bikes over to a Keihin and the tuning window is indeed much wider on a Keihin than a Mikuni. It was not by choice that KTM had to switch to the Mikuni. It's the elephant in the room as Keihin may not have agreed to continue to supply their carbs at the previous price or ordering quantity with TPI coming out in the immediate future. Prices were probably the big factor as Keihin probably wanted a larger ordering quantity to meet KTM's budget for the 2-stroke bikes. I'm not here to debate my jetting skills or put anyone else's in question, but I assure you I have scotchbrited my share of "custom" needles to try and make the Mikuni match the stabilty of the Keihin. I couldn't. The Keihin Airstryker also produces significantly more torque on the front side on the dyno compared to a Mikuni in all the swaps I did. The bike also sounds noticeably different and barks more with the Keihin. My butt dyno also backs the above findings as well. What scares me the most about the Mikunis is how much they differ from carb to carb in performance. One person's Mikuni runs well and the guy next to him with the same bike/fuel/premix is out in left field. At the end of the day I don't blame anyone for shit-canning the Mikuni and going to a Keihin. Yes, some people have good running Miks but I see a larger percentage of people who make the switch and never go back. Having said that, I'm still baffled as to why Yamaha specs the Mik on their 125. A YZ125 with a 38mm Keihen would be a treat.

|

1/23/2020 12:42 PM

hellion wrote:

AJ, I respect your opinion on the Mik, in fact your jetting specs did work about the best I got it to work. I'm not sure how consistent your weather in Texas is and maybe that's why I've had a different experience than you. Here in New England we get pretty wide swings in temp and humidity. While I could have the bike running perfectly it could then be a blubbering mess in the matter of a few hours. When we finally went to the Keihin this problem went away. I even went back to the old NECJ needle I used to love and it's still the ticket. Steve, I know the JD needles require different mains so maybe that's why you're ok with the 165m, that sounds pretty lean to me though so be careful if you do change away from the JD.

Hellion,
I am in the same geographic area as you, just bought a keihin w/jd kit. Was going to ditch the JD needle and run NECJ needle, 175 main and 42 pilot to start on a 17 250sx.
What specs you running?
I will also say I run the Suzuki 17-62 needle in my mikuni and it seems to run much better than w/stock needles.
thx.

|

1/26/2020 10:27 AM

The 2003 ktm 250sx's jetting is a good baseline to work with as it was the same pwk38as that you have put in yours, that's what I based my buddy's 2017 250sx and it fired up and ran beautifully, and he hasn't complained or rich

|

1/26/2020 10:44 AM

DynoDan22 wrote:

The Keihin vs. Mikuni carb debate goes on and on...... Over the years I have swapped more than a few bikes over to a Keihin and the tuning window is indeed much wider on a Keihin than a Mikuni. It was not by choice that KTM had to switch to the Mikuni. It's the elephant in the room as Keihin may not have agreed to continue to supply their carbs at the previous price or ordering quantity with TPI coming out in the immediate future. Prices were probably the big factor as Keihin probably wanted a larger ordering quantity to meet KTM's budget for the 2-stroke bikes. I'm not here to debate my jetting skills or put anyone else's in question, but I assure you I have scotchbrited my share of "custom" needles to try and make the Mikuni match the stabilty of the Keihin. I couldn't. The Keihin Airstryker also produces significantly more torque on the front side on the dyno compared to a Mikuni in all the swaps I did. The bike also sounds noticeably different and barks more with the Keihin. My butt dyno also backs the above findings as well. What scares me the most about the Mikunis is how much they differ from carb to carb in performance. One person's Mikuni runs well and the guy next to him with the same bike/fuel/premix is out in left field. At the end of the day I don't blame anyone for shit-canning the Mikuni and going to a Keihin. Yes, some people have good running Miks but I see a larger percentage of people who make the switch and never go back. Having said that, I'm still baffled as to why Yamaha specs the Mik on their 125. A YZ125 with a 38mm Keihen would be a treat.

I love the pwk airstrykers design, the two air vanes that concentrate airflow over the jets to improve carb signal are awesome, and I have found always to be an improvement over a mikuni on every swap Iv ever done, which is more than a few. But there is the downfall of the air vanes, is they do restrict a tiny bit of flow at the top end, they may help with air velocity In the low end, but wide open, they do act as a bit of a restriction to air flow, and I have noticed that with every swap done, just that tiny bit of over revv is sacrificed, but for the low end gains, worth it every time. As well, no pro at jetting, it's a dark art with a lot of variables, and takes work to find the correct settings.

|

1/26/2020 10:51 AM

hellion wrote:

AJ, I respect your opinion on the Mik, in fact your jetting specs did work about the best I got it to work. I'm not sure how consistent your weather in Texas is and maybe that's why I've had a different experience than you. Here in New England we get pretty wide swings in temp and humidity. While I could have the bike running perfectly it could then be a blubbering mess in the matter of a few hours. When we finally went to the Keihin this problem went away. I even went back to the old NECJ needle I used to love and it's still the ticket. Steve, I know the JD needles require different mains so maybe that's why you're ok with the 165m, that sounds pretty lean to me though so be careful if you do change away from the JD.

opyguy wrote:

Hellion,
I am in the same geographic area as you, just bought a keihin w/jd kit. Was going to ditch the JD needle and run NECJ needle, 175 main and 42 pilot to start on a 17 250sx.
What specs you running?
I will also say I run the Suzuki 17-62 needle in my mikuni and it seems to run much better than w/stock needles.
thx.

Every jd jetting kit Iv ever had, has always made my bikes run lean, crisp on the stand, lean bog when hot and under load, even tried running their needles in the 5th clip, and my bike would still lean spike on the stand when trying to tune for their excessively lean needle tapers

|

1/26/2020 11:47 AM

15tc150: I do agree on the Keihin giving up a very slight bit of over rev. Very slight but noticeable. I also feel it is well worth it for the gains in torque the Airstryker gives on the front side. I think it was almost a half foot pound of torque on a '04 CR250 over the Mikuni.

|

1/26/2020 12:24 PM

DynoDan22 wrote:

15tc150: I do agree on the Keihin giving up a very slight bit of over rev. Very slight but noticeable. I also feel it is well worth it for the gains in torque the Airstryker gives on the front side. I think it was almost a half foot pound of torque on a '04 CR250 over the Mikuni.

I ran a pwk36as in my 03, and loved it, it felt like it smoothed out the entire delivery, and way better bottom end and roll on, and for what it minused up top, actually made the power feel longer and revved out cleaner than the pwk38 I had in it, felt almost as if it extended the top end because it revved out slower, and blended the extremely potent midrange of the 02-07 Cr250r engine with a more controllable top end pull.

|