Need some jetting help! Yz 125, 2000

[Kyleb15]

Ok, guys I just change my 125 back to stock bore with a sleeve and what not. All of the jets are stock and the needle is in the stock position. Air screw at 1.5. FMF fatty pipe with powercore 2 silencer. When in taking off from the start the power is smooth then when is gets gets ready to go into the powerband it likes cuts out. Its almost like it is jumping out of gear then back again, and again, but rather with the power and not the tranny. If you keep holding it wide open it will finally stay in the band and you can fly. Top end is real nice, I did the chop test and got a nice dry brown. Does this signify being a liittle lean and starving for fuel when it is going into the powerband? Do I need to change the needle? Main? Pilot?
I really hate jetting a bike. Also this thing wont pull the wheel too good I always have to clutch it. It will semi get the whell up in 1st but in second, oh no, it wont even begin to pull the wheel. I rode a brand new 03 Cr 85 awhile ago, man this thing was awesome. It would pull the wheel in 3rd easily. My 125 has away more cc's yet in wont pull the wheel. Can you please help!

 

[ellandoh]

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.

 

[Bullwinkle58]

Ok, guys I just change my 125 back to stock bore with a sleeve and what not. All of the jets are stock and the needle is in the stock position. Air screw at 1.5. FMF fatty pipe with powercore 2 silencer. When in taking off from the start the power is smooth then when is gets gets ready to go into the powerband it likes cuts out. Its almost like it is jumping out of gear then back again, and again, but rather with the power and not the tranny. If you keep holding it wide open it will finally stay in the band and you can fly. Top end is real nice, I did the chop test and got a nice dry brown. Does this signify being a liittle lean and starving for fuel when it is going into the powerband? Do I need to change the needle? Main? Pilot?
I really hate jetting a bike. Also this thing wont pull the wheel too good I always have to clutch it. It will semi get the whell up in 1st but in second, oh no, it wont even begin to pull the wheel. I rode a brand new 03 Cr 85 awhile ago, man this thing was awesome. It would pull the wheel in 3rd easily. My 125 has away more cc's yet in wont pull the wheel. Can you please help!

Is your powervalve working? It sounds as if it's stuck closed.

 

[ellandoh]

my 67 chevelle would do the exact same thing when pulling the tires off the ground.........come to find out tilting the holley at that angle with g's would goof with the floats i would check your floats for stickiness or leaks and specs first since that would take all of 15 min max

 

[ellandoh]

Is your powervalve working? It sounds as if it's stuck closed.

pv would be stuck open since it has no low end and is running good at high r's where theyre supposed to be open

 

[Kyleb15]

Nope powervalve is fine. Top end is awesome. I did have some good results by setting the needle to the first clip. It ran really good. But, the virtually very littel smoke. So I set it back and it ran like crap the rest of the day. but I lived with it. So how can get this thing to run good without sacrifice the lubing. I could also probably go up one higher on my main just give it some more lube. I also tried the turn the air screw till it idles faster thing. Nothing. i cound't tell if it was or not. My carb is spotless, and I am all beack to stock jettings. Basically my probaly lies in the needle. I guess it is just too rich a mid throttle. Thanks Also does any body else have suggestion has to how to set the air screw?

 

[ellandoh]

Nope powervalve is fine. Top end is awesome. I did have some good results by setting the needle to the first clip. It ran really good. But, the virtually very littel smoke. So I set it back and it ran like crap the rest of the day. but I lived with it. So how can get this thing to run good without sacrifice the lubing. I could also probably go up one higher on my main just give it some more lube. I also tried the turn the air screw till it idles faster thing. Nothing. i cound't tell if it was or not. My carb is spotless, and I am all beack to stock jettings. Basically my probaly lies in the needle. I guess it is just too rich a mid throttle. Thanks Also does any body else have suggestion has to how to set the air screw?

once it is warmed up there should be no smoke. if i were you i'd drop my pilot jet by at least one size......youre running so rich the a.s. cant even lean the circuit. then tune the a.s.
if it then cleans up some on the very low end ,drop youre needle again if it then rips , do the plug test, do what needs done to your main and keep fine tuning...dont be scared the main jet is the only circuit to worry about a lean condition thats why you do plug tests, the other circuits will ping or bog and surge from leanness. if you dont see the grey ash of death which is aluminum melting youre golden :) .
when you do get it dialed in dont switch premix or ratio.

 

[Kyleb15]

Are you positive that this thing shouldn't smoke. Hell thats what lubes the engine. Thats why they call them smokers. I am gonna try going up on the main one and down on the pilot then put the needle back. Thanks

 

[ellandoh]

you see any smoke out of jbs rc or cr bikes?
they smoke while theyre warming up or after you been putting around for a while and get back on it. seriously use the search function you will find alot of info as far as smokers that is from i would guess thumpers going from 2stroke to 2smoke to smokers hell i even make fun of the name that takes the fun out of it for them.
burning smokeing oil doesnt lube anything, its the mist that is atomized when it is sucked in the crankcase

ask around if youre still skeptical hell paste the jet article in a poll thread and ask if it makes sense

 

[#1YZ125RACER]

my 2000 yz has some jetting problems as well if i have no rpms on the engine and quickly give it full throttle it will bog....if i rev slightly then hit it it really rips any suggestions...by the way it has no problem popping up in 1,2,3,4 prolly even 5 but thats way too fast to be on one tire

 

[viking20]

Guys , read the excellent advice from Ellandoh , its all in there !
No , a 2-stroke is not supposed to smoke when ridden as intended , or drip oil !

 

[Kyleb15]

Yz125 what jets do you have in yours?

 

[ellandoh]

my 2000 yz has some jetting problems as well if i have no rpms on the engine and quickly give it full throttle it will bog....if i rev slightly then hit it it really rips any suggestions...by the way it has no problem popping up in 1,2,3,4 prolly even 5 but thats way too fast to be on one tire

try playing with your air screw a little then do a plug chop test it sounds like your main is a little fat.

 

[#1YZ125RACER]

i'm not real sure what jet is in mine i know it has had the carb jetted...but i bought it used so i dont know the specs

 

[ellandoh]

i'm not real sure what jet is in mine i know it has had the carb jetted...but i bought it used so i dont know the specs

all jets have tiny and i mean TIIINYYY numbers stamped on them somewhere ,when you find that #, get a couple in the direction youre going after and 1 in the opposite direction if its possible youll be in for any temp or elevation changes also know what kind of CARB is on your bike when you call your parts guy its amazing how hard it is to get a knowledgable parts rep. who will actually know or ask what carb it is also alot of them dont carry anything but main jets so dont go for the "they come in packs of 5" they say "then do you want me to order them from the dealer" like its a fortune i dont get it its ...............$4.85 or i can buy 5 at 4.10 a piece. :think: