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KX 250 jetting??

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Hi I'm new to dirtbike maintenance but I understand that jetting has a huge impact on a bikes performance.

So my question is how do I know my bikes jetted right or how do I rejett it if it's not right?

I'm riding a 07 KX250 the previous owner was a inexperienced rider also so i wana make sure it's jetted right.

any help will be appreciated :smirk: thanks!

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Oh man no help for the new fella? I was you like 2 months ago man, don't let it intimidate you. Everyone has a different temperature and altitude they ride @ and the temp atleast is always changing especially this time of year, so you have to make minor adjustments here and there, and especially on the air screw. Adjusting it can be a day to day thing. You're basically need to learn the difference between lean (not enough fuel) and rich (too much fuel) how each sounds. In winter bikes tend to go lean because of atmospheric conditions, so this time of year is typically when you need to go richer on your jetting, and in summer bikes go richer, so you must go leaner.

What exactly is going richer or leaner? Going richer is buying bigger round jets that can allow more fuel to pass through them, and going leaner is buying more narrow round jets so less fuel will pass through. You have 3 main types of jets in your carb and they each have their own number. A higher number indicates a richer jet, and lower # is leaner.

First off, you absolutely must have a service manual for this bike to find out what # jets your bike needs at this exact time temperature and elevation wise...Then you'll want to pull your carb and remove your jets (all explained in your manual) and figure out what # jets you have in it. This should be the DOH! step as you realize your bikes been running crappy because you need to bump up your jets (richer)

Now being too rich is okay because the worst that can happen is fouled plugs, but being too lean is VERY dangerous because the engine is starved for fuel and in a 2 stroke that means less fuel AND oil, so the engine gets extremely worn when you run it under these conditions.

Now think of your jetting from bottom to top end power.

-Your pilot jet controls your ide and just barely blipping the throttle performance.

-Your actual jet needle controls your contrast of the mid range power. You change the jet needle itself or you can change it's position on the clip up or down to rich or lean it out.

-Your main jet controls that WIDE OPEN throttle and is very important you get this one right as when wide open is when being LEAN is crucial to engine failure.

I'll give ya a link here for some more reading but honestly man that service manual is CRUCIAL.

http://www.thumpertalk.com/forum/showthread.php?t=1026116

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Believe it or not man this helped out a ton!

I will definently need to get a service manual because I havn't torn into a dirtbike since I was like 13 ten years later I seem to have forgotten what the inside looks like.

I live in Missouri so it's pretty humid and there's crazy temperature swings throughout the year so I'm going to have to get the hang out this.

What exactly did you mean by, "DOH! step?" Thanks a ton man! I was fouling plugs pretty bad during the end of the summer and my bike was smoking pretty bad people were saying I was running too much oil but I was going by the manufacturer recommended ratio 32:1 I believe.

I guess the carb jetting could of had something to do with it because now it's winter and the bike only smokes when I've started it up and not very much after it warms up.

Thanks again man!!

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Believe it or not man this helped out a ton!

I will definently need to get a service manual because I havn't torn into a dirtbike since I was like 13 ten years later I seem to have forgotten what the inside looks like.

I live in Missouri so it's pretty humid and there's crazy temperature swings throughout the year so I'm going to have to get the hang out this.

What exactly did you mean by, "DOH! step?" Thanks a ton man! I was fouling plugs pretty bad during the end of the summer and my bike was smoking pretty bad people were saying I was running too much oil but I was going by the manufacturer recommended ratio 32:1 I believe.

I guess the carb jetting could of had something to do with it because now it's winter and the bike only smokes when I've started it up and not very much after it warms up.

Thanks again man!!

I knew it would help because I literally just learned about it, so I know exactly how to explain it to someone else that's new to it. What I meant by the "doh!" step is *if your bike is not running right (why you made this thread) then the doh step is when you look @ your jets in your carb and then see what you're supposed to have (service manual specs), you'll say DOh! (homer simpson style) and say OH, this is why it's been running lean etc. etc. cuz it's calling for a richer main and pilot jet! (just for example)

We also have some good temp swings here in Memphis, but as long as you aren't changing altitude alot, then your jetting swings won't be so often. Like if you mainly just ride @ 1 elevation, you'll only have to jet your bike for summer or for winter, and maybe a spring/fall jetting but that's it, compared to someone who goes and races in Colorado, then down to Florida, will have huge jetting changes.

For example in the summer I run a 175 main, 55 pilot, and the needle clip in the middle (3rd position) and in the winter I run a 178 main, 55 or 58 pilot and needle clip still on the middle 3rd position or maybe the 4th...If it gets down 20-40 degrees is when the 58 pilot and the needle clip goes down a notch to 4th (richer)

This explains exactly why your were fouling plugs in the summer. Don't ever change your ratio from 32:1 that is PERFECT...People often try to blame a jetting problem on a mix ratio, when in actuality the mix ratio always stays relative. It's all about jetting! See in the summer, jetting goes what? RICH...Rich = more fuel = blubbering sound = bogging = fouled plugs, etc. etc. and so you would need to bump down a size on your jets (leaner) based on your service manuals specs!!

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Awesome! Yeah when the guy was telling me to changed my gas/oil ratio from the recommended dose I was very leary and didn't agree.

I guess I'm going to look into a service manual to figure out where this needle clip and pilot are and what I need to run them at.

I really appreciate the help man, you cleared everything up now I just need to tear into a service manual and the carb but at least I understand jetting now!! :smirk:

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this thread has been very helpful to me to as this is what my bike is doing ! thanks a lot , would this also make it hard to start the bike ?

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It's truly my pleasure guys! I was lucky enough to get on here and some guy had ordered the download of the manual for my bike and it still had 72 hrs of download, so I got mine for free. It's great to have on the computer, but NOTHING beats a good old real one to have in your hands in the garage, so I've still gotta order one of them.

Make sure you get the OEM Kawasaki, Yamaha, Honda or whatever manual and not the Clymers or Haynes. Those are okay to get later, but you want the OEM for sure.

If either of you have any questions feel free to PM me anytime.

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would my bike having the wrong jetting make my bike hard to start and go pretty crap untill its warmed up , and then it runs fine ?

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this thread has been very helpful to me to as this is what my bike is doing ! thanks a lot , would this also make it hard to start the bike ?

Dave, something like a clogged pilot jet could cause your bike to be hard to start...but you also need to take out your reeds and make sure there are no gaps or cracks in them, like mine had! The whole black section you see is a full crack

IMAG0563.jpg

My bike recently got a little harder to start and had a surging idle, and all kinds of other little things. Turns out I had a dang crack in one of my reeds!!! I bought the boyesen dual stage pro reeds ($40) and a new reed cage gasket, and the bike starts first kick cold everytime, but we also just did the top end and all kinds of stuff, but you need to take your carb apart and clean it really well. I would just replace your jets instead of trying to clean them since they're only a few bucks a piece! Try that and if it still is, I'm betting your reeds are worn out, but those are super cheap too! $40 :smirk:

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Ok so I realize this question may differ depending on how hard you ride and prolly other factors but how many hours would you expect out of reeds for a KX 250? Sorry for the newb question :smirk:

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Haha here ya are...Man, you've got to learn that you can't really rely on hours with things like this. It's like looking at your oil, and knowing to change it when it turns dark, or looking at your filter. I clean and oil my filter every ride, but some rides it needs it and some rides it don't, but it still gets done. Reeds are the same way. You need to get your service manual and find the service limit on the gap. Mine is something tiny ridiculous like no more than .02mm gap of the reed off the reed block. So they basically need to be sitting flush without any cracks, bends, or anything.

These bikes and our riding conditions are the opposite of consistant, so just never rely on hours or whatever just gotta physically get in there and check it out! Hell pull your reed block out and take some pics, then post it up and you may get some better responses. Tough crowd here on TT.

Worst case scenario, you're forced to buy brand new reeds ($40) and go with Boyesens for sure.

If your bike has any sort of problem starting like hard starting or taking more kicks than usual, or surging idle, it's your reeds. Google the symptoms of bad reeds. They can cause ALL sorts of crap because they funk up that airflow if not perfect.

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Once again, huge help guys, yeah I just finished reading up alot on reeds because I didn't even know exactly what they were, now I feel stupid because I didn't realize there were basically just an intake valve.

Yeah my bikes been taking more kicks than usual even after it's warm so I was thinking my reeds are going out or my plugs getting fouled up from the improper jetting.

Thanks again for spending time to help the unexperienced!! And BlackCR I've gotten way more help here than any other forums, I'm pretty stoked lol

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hi mate checked the reed's and stuff all good , started the bike today and there seems to be a slight hang idle , when i rev it it seems to take a while to get back down what would cause this drawing air or jets ?

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hi mate checked the reed's and stuff all good , started the bike today and there seems to be a slight hang idle , when i rev it it seems to take a while to get back down what would cause this drawing air or jets ?

No a hanging idle means that your jetting is too lean. When it gets colder (temperature) the jetting needs to be richened up (more fuel/oil) and when it gets warmer they need to be leaned out (less fuel/oil)

Do not try to do this by changing your premix ratio. That should always stay @ 32:1..

First off try turning in (clockwise) your air screw until the problem goes away. Now the "air screw rules" are, if you are less than 1 turn from fully screwed in, you need to go up to the next larger (richer) sized pilot jet. So if you turn it to 1 turn out and it goes away you are okay, but if you have to go to 3/4 or 1/2 a turn from fully closed then you need to order a bigger pilot jet.

Now, this is for leaning out (counter clockwise) the air screw (summer) If you go more than 3 turns out, you need to go to the next leaner (smaller) pilot jet.

Make sense? Right now your pilot jet is too lean. So you can try to richen it up by turning in the air screw, but if you have to go more than 1 turn out from fully closed, you needa' get to ordering the next size up!

Let us know how it goes.

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I'm trying to learn how to jet too. When you jet for the winter and summer do you change the pilot main and needle position, or just one of those?

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I usually can get away with going bigger on the pilot and main, but it all depends on where your jetting is at to begin with. Your jetting will vary with temperature, humidity, elevation, fuel type, etc.

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Most of the time (depending on your elevation and temp swings) You just jet for summer, winter, and spring/fall. Spring and fall are grouped together because they are very similar temperature wise. So mainly 3 jettings. For me, my 98 cr, I only have to change the main from 70 degrees down to 40. Below 40 I have to change my Pilot and lower my needle clip position. The most often changed jets are definitely just the Pilot and Main jet (and these will yield the biggest results) and lastly the needle clip position will be changed, but usually only in the most extreme low of winter (for me) once it gets below 40 degrees (which is when I won't be riding anyway) is when it calls for me to change my needle clip to the 4th position, instead of dead middle (3rd) .

But basically I run a 175 main jet all year long, until it dips below 50 I am now running a 178 main, stock everything else. Then once it dips below 40 the manual calls for a step bigger pilot, but for the most part temperature range wise I am stock jetting.

The air screw is something that needs to be adjusted DAILY to receive the best idle and throttle response.

You just always want to make sure that your jetting is never too lean. Too rich is okay because you just foul plugs and over lubricate your motor. Too lean is always dangerous because you're starving your motor of fuel and oil, which leads to seizures and blow ups. So in winter time, it's critical to fatten up (richen) your jets, and in summer time when the jetting automatically goes richer, it's not as crucial to lean it out, although your bike will run better.

Basically you really really need your bikes OEM service manual. It will have your jetting chart, and that's where you really learn what's needed and how your carb changes due to the atmosphere. People that ride/race in one place elevation wise will have to change jetting the least. People that go ride in Colorado one weekend and Florida the next, will have HUGE swings in jetting changes, due to elevation.

Make sense?

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