2000 YZ125 Trouble

[Jim X]

Hey guys great forum you have hear, this is my first time posting and I need some help. I just bought a new bike, a 2000 yz125. I didn't get to test ride it much but decided that at $1300 I couldn't pass it up.

Here are my observations:

Low to mid rpm the bike runs fine and has torque and power through the beginning stages of hitting the power band. But as the bike accelerates further it starts to sputter, as in: blaring along about 3/4 throttle through second gear, it starts out like a 2stroke not much torque in the lower rpms. Then picks up some speed and never bogs and starts to hit the power band, just after it does this it starts to sound like it is missing. Like....broooppp, braaaap bap bap, baaaaaaaaaaap, then shift.

I did notice that the spark plug hook up was a little loose feeling and was wondering if this is the problem. But then the spark plug (BR8ES) fouled after about one hour of riding. I was in a smaller area and only able to hit 3rd gear on the longest straight, most turns were 1st-2nd gear berms.

Since the spark plug fouled (a wet foul by the way), I figured that it could be jetting (too rich on the main jet, maybe?). I am not sure as I have not worked much on dirt bikes, most of my mechanical knowledge is from autos. Anyway, I have taken apart the carb (stripped off float and pulled the slide dealie, basically) and I noticed the huge needle in the slide. I am guessing this is the main jet and looked for a way to adjust it. The only thing I can come up with it that you need a special tool to do it because I see no way to get inside the slide and it is such a tight space. I have obviously removed the throttle cable to get at the needle but still see no way to actually get to it. Am I on the right track or just wasting my time with trying to adjust the jetting?

ANY HELP WOULD BE APPRECIATED, any info at all I'm sure will at least help me in some way with figuring this out as I am a n00b to dirtbikes. So whatever advice you have, I'll consider it, whatever the advice.

Also, anyone have any tips for changing a brake side fork seal on this model?


Thanks for all the help, in advance!!

[Jim X]

If anybody has any diagrams of the carb, that would be greatly appreciated, as I don't know for sure what all the adjustment screws and such are, thanks.

[Aus_Rider_22]

G'day! Welcome to ATM!

I have the same bike. Try a BR9EG sparkplug. It works very well. If you can spend the bit extra you should go for the Iridium version of this plug. (BR9EIX)

I would take your bike down to a reputable dealer have them give the bike a once over just for peace of mind. Also, while your bikes there, I would get them to check out the jet sizes. My YZ was jetted way lean. So we reverted back to the factory sizes and it works great now! Maybe a bit rich but I might go one size leaner on the main soon.

Here is a diagram of the carburetor (sp?).

[Only registered and activated users can see links. ]

The stock jetting for the 2000 model is

Needle: 6DHY53-75
Pilot Jet: 35
Main: 350

And try air screw about 1 1/2 screws out.

Good luck! Let us know how you go with your bike. THey are great bikes!

[Jim X]

Thanks for the link, but it seems there is something a little screwy. When I click on the carb link, it shows me the diagram but some of the numbers dont match up, like #18 and #19 are the o-ring and plug but #24 is definately a gasket and not the main jet!! It's alright though, I can figure most of it out, and with that exploded view, I have now figured out how to take the needle out and adjust where the clip is...now where should I set it? I think it's at 3 right now but I'm not sure how to judge it. Where do you count the clicks (ring slots) from, top or bottom?

Also, how much smoke should be coming from the exhaust. There is a heathy blue puff right now at 32:1, should I try 40:1 or wait 'til I get the jetting sorted?

Man, I sound confused. Like this--->:

[Jim X]

Alright, I think I might have something figured out a little. If my engine is rich, i.e. getting too much fuel, then I can adjust the needle down a click and the needle will actually be sticking further down into the main jet, in relationship to the throttle right? Like, it would be letting less fuel in at 1/2 throttle for instance, when the needle is four clicks up from the bottom instead of 3 clicks up. Is my thinking correct, anyone?



Ok, it's 4 a.m. I'm going to bed, I'll look forward to your suggestions!

[Aus_Rider_22]

I would stay at 32:1 with a good quality synthetic oil. It sounds like you are on the right track with the needle. Some of the jetting gurus should come out of the woodwork soon and can offer you some better advice, as Iam a beginner at jetting.

Good Luck!

[Woody_393]

Yep, you've got the ticket. drop the needle one slot (raise the clip). You count from the top going down. So if you are 3 from the top, change it to 2 from the top (Leaner, exactly as you mentioned).

However, My thoughts are that your sputtering (if jetting changes don't cure it) is the reeds. When reeds get worn/chipped etc, they will do exactly what you are describing.

The reeds sit between the carb and the engine

carb -> carb boot -> reeds -> engine

Take them out and have a peek at them to see how they look.

If they are chipped/cracked or don't close all the way (when just sitting there) you need new ones. Which if you are looking for some more power, Vforce makes a very good reed block that will boost the power for around $140 or so. Otherwise, replacement reeds are ... hmm... not sure, I have a thumper , but I think around $15 - $30 depending if you get OEM or some exotic OEM replacements.

[Woody_393]

Here's some replacement Boyesen Power Reeds (replaces OEM)

http://www.motoworldracing.com/boyes...powereeds.html

[Woody_393]

Here's a good read on Reeds. The page talks about sleds, but it applies regardless.

http://www.snowtechmagazine.com/arti...qa/reedqa.html

[Aus_Rider_22]

To add to the links above, here is one of the recommended jetting specs of stock and recommended after installing Boyesen reeds and cages.

[Only registered and activated users can see links. ]

[Jim X]

Wow guys, thanks for all the help. I am actually at a family reunion for today and wont be home until tomorrow, but before I left I dropped the needle a slot and threw everything back together. While it was all apart I looked at the reeds and I didn't see any cracks or chips, but they do seem kind of weak, like they don't close hard and are really easy to lift. They looked like they were sealing alright, but like I said, they looked lax. I didn't get a chance to ride it but when I get back (sunday evening monday morn) I'll try to give 'er a ride if it isn't too dark. If it does the same thing, then it looks like I am getting new reeds. It looks like though, from what I have been reading, that putting boyesen reeds on the bike would make it run rich so I will probably be keeping the leaner setting to test out the new reeds (that is, if I get them )

Thanks for the recommendations guys, I've heard real good things about boyesen reeds, so thats what I'll probably get, but I would like reeds that fit the stock cage for monetary conservation reasons. Do the reeds you sent me a link of fit the stock reed cage?

Like I said, thanks for all of the info!!

[Jim X]

Originally posted by aus_rider_22
G'day! Welcome to ATM!

The stock jetting for the 2000 model is

Needle: 6DHY53-75
Pilot Jet: 35
Main: 350

And try air screw about 1 1/2 screws out.

About the air screw, is that the adjustment right at the airbox side of the carb? Right by the boot. I have turned this screw in and it was set at only 1 turn out, so I'll have a go at adjusting that another 1/2 turn, that is, if I remember.



I also, noticed that the subframe is bent to the right a little bit (maybe 1/2 inch), any tips on bending it back?

[yamracer58]

Here is spanks jetting guide. it is good to have around if you are a begginning jetter.

Just follow this and you can re jet your carb.
i would also print off the digram of the carb.

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 power band. 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.

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.

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.

Before you start the jet testing, Install a fresh plug. Warm the bike completely, and shut it off.

As already stated, start with the pilot circuit. Turn the air screw 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 idling. 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 air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw 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 air screw for the fastest idle, it's time to tune the air screw 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 air screw 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 air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too lean 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.

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.

[Jim X]

Wow, thanks yamracer58 and thanks spanks for that guide. I will undoubtedly use those tips, taught me some things definately.

Guess what! The bike is now running just slightly rich!! And it no longer sputters. Looks like dropping the needle that one clip did the trick, now looks like I just need to fine tune with the air screw. Thanks guys, this bike is running great now, but sadly I only got to ride for an hour today. My riding buddy re-broke his clavicle, shame. Now I won't get to ride as much but will get to work on it a little more. A new throttle assembly is looking good right now, also new radiator shrouds and a rear fender. Probably some new graphics to beautify the new plastics and a seat cover to cover my hole in the seat. I also need to weld the shroud support back on the radiator, the previous owner broke it off and only zip-tied it back on. Then of course, bending back the subframe.

I want to thank you guys all again, for all the help. I don't know if I would have known what to do with out you.