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creeks&peaks

Mikuni Carb Fixed and Kickin A$$

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So I got my 2018 KTM 250XC about a month ago, and the guy I bought it from had never done the proper fixes on the mikuni carb. I did lots of reading on this site and bit the bullet right away with a JD Jetting kit and jet block gasket kit. I live at 8000’ and ride up from there. All I can say is that the last couple rides after I fixed the jet block gasket, needle seat o-ring, and got the proper needle and jets in there, it’s a beast. I think this carb got a bad rap cause people didn’t fix the gaskets before playing with jetting. Anyways, just very psyched on this bike and how fun and nimble it is. 

E88D1384-18F7-49E2-9493-6630DA90182B.jpeg

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Cool!  Glad you got the bike working well.

What did you end up with for settings?

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The settings are: Red JD Needle with clip in 2nd position. 
PJ: 30

MJ: 380

Air Screw: 1.5 out

There is a little bit of spooge after a 30 mile ride, but at this point the bike runs so well I don’t really care. Could also be the packing is so saturated from the previous owner running it so rich, so I may get some new packing for the silencer and see if that helps. 

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It will be fine till the weather changes 

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Yea, I got the same results after I had to fix a brand new Mikuni carb, and when the weather changes you will readjust it several times, but's that not a big deal. We should not have to do this to our new KTMS.

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1 hour ago, jeffrey sisti said:

Yea, I got the same results after I had to fix a brand new Mikuni carb, and when the weather changes you will readjust it several times, but's that not a big deal. We should not have to do this to our new KTMS.

What sort of changes do you make with fluctuating weather? Is it mainly air screw adjustments? Or are you rejetting?

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Buy a Keihin PWK Air Striker with Stic block and JD red needle. You’ll never have to mess with it again. I ride at sea level and up to 9,200 feet with out changing a thing other than the air screw. Even then it’s only about 1 turn max. 
otherwise the Mikuni will have to be re jetted all the time. Just the way it is. 

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Last thing you have to do is fix the power valve o-rings. It completely fixed the remaining issues with jetting on mine. Now winter to summer and still runs the same just like my older KTM with a Keihin carb, no changes. Elevation no prob as well. Look up 2 stroke leak test and see for yourself. Won't hold pressure for nothing. KTM talk has some good threads on it too.

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I've had mikunis going back 20 years ,never had one that wasn't sensitive

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8 hours ago, creeks&peaks said:

What sort of changes do you make with fluctuating weather? Is it mainly air screw adjustments? Or are you rejetting?

it's the little brass screw at the back of the carb, yes that's the air screw and you should become familiar with it, a small screw driver fits in you tool pack nicely. And I can see having to adjust that from time to time. But we should not have to put a jd jet kit in every KTM 2 stroke. Lets remember "ready to race"

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1 hour ago, mog said:

I've had mikunis going back 20 years ,never had one that wasn't sensitive

🏆 one can lead a horse to water but...

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8 hours ago, creeks&peaks said:

What sort of changes do you make with fluctuating weather? Is it mainly air screw adjustments? Or are you rejetting?

If your not picky about how your carb runs you’ll be fine. If you do then You’ll be making jet changes. It’s just a finicky carb. Air screw isn’t enough bud. But the Mik is a good carb. It just needs adjustment on weather changes and altitude changes. Been there done that LooL. Won’t look back. 

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On 7/29/2020 at 5:23 PM, jeffrey sisti said:

it's the little brass screw at the back of the carb, yes that's the air screw and you should become familiar with it, a small screw driver fits in you tool pack nicely. And I can see having to adjust that from time to time. But we should not have to put a jd jet kit in every KTM 2 stroke. Lets remember "ready to race"

We've had several KTM 2-strokes, and never had an issue with the carbs except some jetting changes, which is normal for any carb. The ready to race motto is fairly true if you're actually racing and have it wound up all the time. Not many people have it opened up while riding in the woods.....

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On 7/28/2020 at 11:41 AM, mog said:

It will be fine till the weather changes 

They are sensitive to temp, but very easy to just add the extended air screw for 2 second easy adjustment on the trail as you need. My Mikuni's on the '17 and '19 work (ed) great.

 

Joe

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On 7/29/2020 at 3:23 PM, jeffrey sisti said:

it's the little brass screw at the back of the carb, yes that's the air screw and you should become familiar with it, a small screw driver fits in you tool pack nicely. And I can see having to adjust that from time to time. But we should not have to put a jd jet kit in every KTM 2 stroke. Lets remember "ready to race"

'17's were definitely off the mark and the JD kits do wonders. Not completely necessary for the 18 and 19's as long as you notch the slide and do the well documented mods.

Joe 

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Then  you have to change the needle clip and main from winter to summer , the in between weather gives you problems as well 

 

It's simply a ok carb , it's not a good carb ,you can make yourself believe it ,bit it's denial , the keihin will make you happy ,the mikuni will make you happy sometimes 

 

For me the pain isn't worth it 

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I tried the Keihin on my bike and saw no advantage to it.  I found the Keihin just as finicky with the air screw.  

For me the stock 38mm Mikuni has more low end torque than the 36mm Keihin did.  That was surprising and perhaps the Keihin needed a different needle but I still think the Mikuni works well.

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6 hours ago, trailmeisterjoe said:

They are sensitive to temp, but very easy to just add the extended air screw for 2 second easy adjustment on the trail as you need. My Mikuni's on the '17 and '19 work (ed) great.

 

Joe

Yup, on my 18 I eventually got to three baseline settings for needle/main

10+ C

-7 to 10+C

Below -7 C

Anything else was just a screw fiddle. Carb worked great was just subpar quality, and at least my KTM was shipped with Jets that were irrelevant

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