02 KX 125 Carb Tuning... 10,000' Tell me what you think

[LA Sledboy]

02 KX 125 Carb Tuning and other Mods... 10,000' Tell me what you think

I posted all this on another thread but I figured it get more attention here. I know some of this stuff might shock you guys especially lower sea level guys but Im just putting out information and asking for your opinions. I ride sleds and tune them. Same basic principles. My sled is bored, ported, piped, new heads, clutched (different than bikes in that department) and tuned to perfection. Im just saying this to show that I have the ability to do some tuning. Anyway heres what I was thinking for this 125 Ive got.

Ive got a 02 kx125 w/ fmf pipe and silencer. Stock pilot and clip position (as far as I know-- it didnt appear the previous owner would know to do much with these things) but I think it was a 350 main in when I got it. Anyway I dropped it to a 300 main right now w/ no problems. Lean and mean. Still having low end problems. Is there a fuel/screw on these carbs or am I missing something? Im a sledhead so bikes are kind of new to me. Oh and before you say 300 WOW!! I ride it up to 10000'

Actually it might have been a 400 main I cant remember (All I know is the main was what FMF recommended for sea level on this pipe... 300 now anyway). Anyway like I said Im a sled head but its all the same. I was looking over my carb and didnt see the usual air/fuel screws Im used to. Figured it out though the idle screw is the one with the locknut which Ill be having to change since it wont idle then Ill turn the air screw out 1-2 turns and see how she livens up. Not that the performance is too poor at the moment. Im also going to drop the needle a clip or two (probably two). Asking for trouble eh? :D Oh also Im thinking about running a br8 or br7 instead of the br9. Like I said before guys Im at 8000' where I start and its up from there.

Even though this is a MX bike its not WOT all the time or much at all. The throttle is probably in the middle the most and I am a clutching fool on these high rocky rutty mountain trails. Great place to learn how to really ride one of these bikes though right ;)

Thanks for all comments in advance.

 

[ellandoh]

this is what i found on the net that i really liked the part about the a/f screw :cool: i think your idle prob is the pilot not the idle screw


Spanky's jetting guide:

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.
Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.
The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.

 

[LA Sledboy]

Im having more thoughts so help me please. Like I said in the first post the idle will be corrected and the needle dropped a clip or two. I am also thinking now of just changing out the slow (pilot) jet. I think stock is a 50 so im thinking drop that down to 46 or lower (whatever they make) not sure. The Main is supposed to have been a 340 on the stock bike but it was a 350 when I pulled it originally. I guess when the first owner had the FMF put on they put a 350 which was the jetting for sea level so I dropped it down to 300. So anyway Im thinking 300 main like it is, 46 or lower slow jet, drop needle 2 clips.

Now onto other mods. I put on a 120 / 90 x 19 rear tire on which is about as big as you could go Im thinking. Its larger than the stock tire definitely and definitely likes eat up some ground. Of course its probably harder for the engine to turn. Stock rear sprocket is a 49. Would changing this to a 53 or so help me out. I would think it would. Keep in mind again Im a high mtn trail rider. Im not racing so a little top end loss is nothing if I can gain bottom end torque. I could drop the front sprocket as well to an 11 or so but Im not a fan of small sprockets (In the sled world we dont like to go much smaller than 19 tooth top sprockets for fear of breakage, course the hp is quite higher)

Also whats with changing the flywheel weights? Tell me about this please.

All tips and advice are appreciated. Its an 02 KX 125 ridden on rocky, rutty, steep, slippery, tight, wooded, HIGH altitude mtn trails and climbs.

 

[binthedirtnow]

I have a 2002 kx 125 also and I went to this site.

http://www.teamgreennews.ca/techside.html

On this site there's a bunch of good articles including jetting for your bike. Well, I went with their recommendations and it was almost perfect. I went one smaller on the pilot and ended up one bigger on the main just for safety because I trail ride the bike. I'm running 40:1 redline synthetic with very little spooge, too. Now, if you have the owner's manual for the bike it shows how to calculate for the correction factor based on altitude. So, what I'd do is use the team green settings corrected for your average altitude and you should be close. The airscrew is on the side of the carb next to the idle screw. Yes, you can tune the idle on the TMX 38 - use the airscrew to obtain the best throttle response, then set the idle where you want it. I always need to set the airscrew each time when I go out for air density changes, but everything else I leave alone and the bike always runs awsome. So, that's my recommendation to you.

Also, I'm using 12/51 gearing with some success, though I may change that to 12/52 or 12/50 after installing my 9oz flywheel weight, but I think 12/53 would be too screechy.

 

[LA Sledboy]

12/49 4.08
12/53 4.41
11/53 4.81
10/49 4.9
10/53 5.3

Maybe switch to 10/53 :D

Kidding. Thanks for the advice. I have looked in the manual to figure the correction factor although its somewhat different since I have a different pipe and silencer on it. I think stock is a 50 pilot and Im going to go down to a 45. Ill keep the stock needle and raise the clip.


Oh also remember I have that bigger tire so 12/53 might be the ticket.

 

[LA Sledboy]

Ok also would a hotter plug be good for the higher altitude and lower rpm operation to keep more heat in the pipe/engine? Like I mentioned switching from a br9 to say an 8 or 7 even.

 

[binthedirtnow]

Sorry, I forgot to mention I have an FMF fatty and an fmf turbine core Q silencer, so the jetting I'm using would probably be close for you too, after the altitude correction of course. Stock plug is a br9eix but a es or eg will work too and they cost less. Right now I'm using a br9es and it is great. If the jetting is good, I don't think you'll need to go with the hotter br8 plug. I tried that and didn't get any benefit other than the possibility that if anything ever changes for your jetting requirements, you could hole your piston with the hotter plug, so for safety I stayed with the br9 and it runs awsome with team green jetting, but with a 40 pilot instead. That's the most important thing for trail riding, get the pilot circuit correct. The recommended needle clip and main jet seems to work great with the slightly leaner pilot, which helps for closed throttle riding. Also, use a good premix oil at 40 -44:1 so you don't spooge up with recommended jetting. Of course you could just jet differently and run 32:1 oil ratio, but the team green settings work!!! So, why waste time chasing jetting!!! Use the settings and recommendations they spent countless hours testing to establish, make the corrections for your average temperatures and altitude and go ride!!! I did a little math and came out with 320-330 main; needle clip 2nd from top, 40-42.5 pilot for an average altitude of 9600' at 68 degrees F. That is a correction of the team green settings, so maybe a 35 pilot instead and you should be close!!
About your rear tire: is a larger diameter or just wider? I think you'd like 12/52 or 12/51. Is you gearing the stock 12/49 right now? If it is try the 51t rr, it makes a big difference on the trails. I'm actually considering going back down to a 12/50 to have a little bit more headroom for first gear and wider gear spacing for a little less shifting, but I need to go rode with the fww first to see.
As far as the fww, I don't know yet what it will do exactly. I've been reading stuff for a few months now and I've decided to give it a try because most opinions seem to think it helps smooth the power, reduce stalling on hills, hillclimbs easier, more traction that is easier to control, etc. Won't add any extra torque, just change it's character to be more trail friendly - we'll see.....
One final thing: the little screw for air/fuel mixture is only an "air screw", it adjusts the amount of air for the pilot circuit. The fuel is controlled by the size of pilot jet you have installed, so when you fiddle with the air screw, you're tuning the amount of air to match the fuel flow from the pilot jet.

 

[LA Sledboy]

I went out and pulled the stock 45 pilot and put a 40 in. That was the smallest I could get here in town. I also ordered a 52 rear sprocket and an 11 front sprocket to play around with. Yes I realize thats A LOT more gears but I did increase tire diameter so its not quite as much. I can always go back to 12/52. Anyway I pretty much start at 8000' and go on up from there. Its an almost all bottom end torquer that Im trying to make here (out of an mx bike ;) ). I think it will be interesting. Ill still be raising the clip as well as getting a good idle and air screw position. Ill have this all figured out within the next few days and report on how it does.

 

[binthedirtnow]

I am kinda curious.....You don't have any clearance problems with that size rr tire?? May I'll try one that size next time......Also, I was going to get a 11t at one too but I didn't because I read somewhere that an 11t might cause the chain to hit the case or wind it too tight or something, so be sure you check that before riding and breaking anything....
B.

 

[LA Sledboy]

Well the tire doesnt rub the swingarm or anythign but that guard behind the shock got chewed up. For my style and area of riding though I really like the tire.

Anyway my gears arent here yet so I havent tried them BUT I cleaned the filter and box spotless, put a 40 pilot in, moved the needle clip up to the top clip position, and kept the 300 main. Air screw at the moment is 1 1/4 turns out. bam seat of the pants big difference. I thought it was pretty decent before except for a low range bog, I never thought the midrange was that bad which obviously there was much more of one than I realized. Now its in the powerband before you can blink no bog anywhere. Power instantly on demand.

We will see how the new gears work out. I didnt order a new chain. How big can I go without adding links or getting a new chain?

 

[LA Sledboy]

Another question. I havent done a compression check on this bike but if they are up around 180 or so like people say at sea level wouldnt you have to be running race gas down there. I can get away without it maybe at my altitude, which I have been obviously. Sometimes I go down to Cody, Wyoming which I believe is at about 5000' With my new jetting going that much leaner would it be easier to just throw in like a 50/50 mix for safety rather than rejet.

In a sled if I was running that kind of compression up here Id be in need of 50/50 at least.