yz426F lean?

Does anybody have a problem with a stock YZ 426F running lean?



Was anticipating this question at some point. What are your settings and symptoms? Pilot, pilot screw, needle and clip, and main. Yamaha has increased the needle taper in the YZ426 making a more progressive change from lean to rich. The new carb for '00 might need to use a richer clip position (or 2) if you have the EVR needle. The main jet was also decreased which could be to compensate for a richer taper, but may have been overkill if it is a #162. Mike, this was a loaded question, do you have a stock YZ426 running lean?


Mine was lean too. Follow the this discussion list and it will lead you to a happier, healthier bike. I did, bike runs much better, and no more popping on decelleration. Change main jet to a 165 and the pilot to a 48 and you will be all set. Of course unless you live in some super high altitude.


I need advice on 426 jetting.All of my riding is above 5000ft.I have never done any carb tuning, but have read a lot. Carb is stock,162M 42P and "jet needle" is listed as OBEKR-4. I do have a Big Gun sds exhaust. Any advice will be much appreciated.



'00 yz426

'81 490 Maico


I will send you a e-mail to help reduce list traffic.



I sent you a message at ricky1@2extreme.net and the message came back undeliverable. Is this the right address?

Just post it for everyone to see Tim. Don't worry about list traffic. I do all my riding at about 6000.

Again, sorry for the lengthly post. Ricky mentioned that he has not had any prior jetting experience. I hope this helps sombody......

I figured it would be better not to clog up the discussion list with my mumbo jumbo jetting advice. Since you live in elevations over 5000 feet, you should probably run a bigger main jet than us down here in the flatlands. You said you also have a big gun exhaust right? That would lead me to believe that you have to go even bigger on both jets to allow for the more free flowing exhaust. Since you said you have no prior jetting experience, let me start at the beginning. If it gets boring, just skim down until something catches your eye.

The first thing you have to do is figure out the carb you have on the bike. Since I also have a '00 YZ426, I just went through the carb last week. Before that, most of my jetting experiences have been on one of my several street bike experiences, along with some 2 stroke dirt bikes.

Since you have read alot about jetting, you probably know about the three systems in your carb. The pilot jet, (which controls the fuel intake up to about 1/4 to 1/3 throttle), the main jet (which is the top end or 2/3 on up to full throttle), and the Needle "jet" that is the in-between of the two, blending in between 1/3 to 2/3 throttle positions. On our 426s there is also the fuel screw, I think they call it the pilot screw in the manual. This basically controls the amount of fuel we start off at before the "jets" mix air with the fuel to get our desired mist before it is sucked into the motor.

To check the pilot jet , you will find that your motor will be sluggish off the line if the pilot is too rich (too big), or if it is too lean, the bike will pop and snarl when you let off the throttle when the bike is in gear. The popping sound means the pilot is too lean, and should have a bigger one put in.

To see if the Main is too lean, your bike will top off on power much too early, it will not pull smooth at high rpm's. If the main is too big, you will foul plugs alot and the motor will drowned out and bog when throttle is wide open.

The needle is more tricky, after you get the top and bottom down, you might have a "Flat spot" in the power. Meaning it will not pull strong in the midrange. To fix this, you must experiment raising and lowering. Raising the "clip" on the needle will lean it out and lowering the "clip" will make the needle not sit as far down and the bike will run richer. The needle actually sits down into the main jet. Make damn sure you do not ding up the needle!!!! Just the slightest bend in it, could cause the needle to catch up and not fit all the way back in the main jet and you will stick at full throttle. Trust me on this one, I was once on a wooded trail on sandy whoops on a old quad racer when the needle did not go back in the whole. Not a fun experience.

The trick is to narrow your particular problem down so you can correct it. This can usually be done by riding the bike and knowing when it is not running in the desirable way. I am not sure what jets you have in now, but my bike came with a 45 pilot jet, 162 main, and the fuel screw was at about 1 1/2 turns out (from bottoming out). I changed my settings to have a 165 main and 48 pilot, also tuned the fuel screw out to 2 1/4 turns. Now in stock trim it runs and starts great!! Of course I have a stock pipe and running at 500ft above sea level also....................

If I were to run your situation with the pipe and at the altitude you are running at, I would probably run a 170-175 main, a 50-52 pilot, and move up the needle a notch or two. Please forgive me if I am typing too elementary for you, but the main jet is at the bottom of the carb. You can get to it by loosening up the carb clamps, pulling the carb out of the rubber boots and pointing the bottom of the carb out towards the gear shifter. After looking at the set up that holds the throttle cables, I would try to leave them intact while you are changing the jets. You will have to unplug the electrical (throttle position sensor) connection. It is easy to get on and off.

After you get the carb tilted, get a towel or something to prevent the carb and it's surroundings from getting scratched up. This will also help you keep track of any screws you may drop during the whole process. Take the four screws that hold the "bowl" on the carb. The first time you are doing this, keep the owners manual handy. It has quite a bit of info in it for being an owners manual. Like I said the main jet is the one that is in the middle of the carb. It has a hex shape on it and you just put a wrench on it and unscrew it. And screw the new one back in. It will stick out of the bottom. Be careful when you handle any jets, any particles or dings in them will cause you unforeseeable anger down the road (trail).

The pilot jet, you take out with a small flat blade screwdriver, look in the manual to see which one it is. Just screw another one in the same hole and you are done with that. The needle is a little trickier. Once you get the pilot and main jet, dialed in, then you do the needle settings. You might not get it right on the first try, that hardly ever happens. But when you do get it right, you will know it and you and your bike will be happier you spent the time and energy doing it.

Sorry I got a little lengthy. The key is understanding what needs to be done, and the rest is busy work getting it done.

Hope this helped

Let me know


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