2001 Carb


Not to bug you all, but I would like some writen ammunition to pass on to my shop. In my magical 3 months of trying to get my jetting problem fixed on my 01 426, my dealer CO Powersports have now told me that Yamaha says these are very poor trail bikes, specifically that the carburator is to altitude sensitive to have this bike being used as a trail bike. How many of you use this bike for trail riding an what's your take. I still say this is a cop out, I have to many friends between Boulder and Breckenridge with 426's that don't seem to have any trouble what so ever. I would like to be able to share this with the shop.

Thanks, & keep the throttle twisted.


I am in Florida so there's not much altitude change, but I have on several occasions putted around at very slow speeds (1st and 2nd gear) while my wife followed on an XR100 and my 01' 426 ran flawlessly. I also take the bike to an MX track where it has also run flawlessly to this point. Everything is stock in the motor department, even the jetting. I think they're trying to get out of helping you, don't let them off the hook with such BS.

I live in SW Washington and have a '00 426. The engine and jetting are stock. I don't trail ride very often but I have done a couple of rides that go from 500' all the way up to 2700'-3000' and I did not have any problems whatsoever. These bikes should rip on trails-I know that mine does. My experience is that if you hassle a dealer enough with letters and such they will eventually get tired of you and fix what you want fixed.

:) Joe

I've been on rides (with my '00) of up to 8500 ft. elev. with no problems - if anything it's less sensitive to changes in altitude than a two stroke. Other's posting here have had problems with the CDI on their '01 models, which could cause plug fouling.


Holeshot's Page

[This message has been edited by holeshot (edited 08-10-2001).]

Dodger, is your name John? You may know my friend XXXX XXXX at CoPo (formerly Cycles of Boulder, not a Yammy dealer). He called me for jetting advice. I will say it again, your problem is NOT jetting. CoPo is trying to wear you down so you will go away. Anyway, if you are John, you bought the bike used. If not, there is another guy like you dealing with the same problem.

Call me next week and I will meet you out at Berthoud. I will swap CDIs with you and check out some other stuff. Hopefully you can then go into them and see if they will give you whatever parts you need. My number is 303 548-0488.

Look at the postings from the '00YZ426 riders. The '00YZ426 has LEANER jetting that suits higher altitude riding better. The needle in the '01 is richer across the range for MX applications.

Suggest you at least try this jetting before it drives you insane.

#40 pilot jet

1 1/2 turns

OBELR needle clip #3 (part#5NL-14916-ER)

#155 main jet

Lower your float level 1mm below stock height.

Make sure the pump timing screw has a delay so the slide clears before it pumps. Turn the screw in 1/4-1/2 turn to get a greater delay.

The needle I recommended is from the YZ250F and is slightly leaner than the '00YZ426.

The settings have successfully been used by others for high altitude riding. The key is the needle for reduced richness on long downhills. Popping is lean but sputtering is rich. Avoid blipping the throttle like a 2-stroke, it pumps extra fuel when it is not needed or desired.


my 99 yz400 runs like a fuel injected car never ever fouled a plug .after going to a larger pilet jet the bog off idle was gone even if you wacked the throttle wide open from idle . i love mine on the trails


I don’t think you have a jetting problem, either.

I normally ride around 4k feet, but I rode three days up at Rampart Range last month on my ’01 YZ. I ran my old ’00 stock needle (EKR), just a half clip leaner than the one James Dean recommends, on # 3. That was okay but may have been a little rich.

I ran a 40 pilot which was, again, okay but felt like one size too rich. Hey, c’mon, I’d rather ride it than jet it. :D

And a 160 main, but I don’t think I ever actually used that.

My friend’s stock ’01 YZ, before we changed anything, didn’t run that bad at Rampart, it was just little burbly off idle and you could easily tell that the clip position was off by the stutter and stumble that you got when you twisted the throttle. But anything beyond, say, ¼ throttle and it still revved out fairly crisply. This was at 8 to 9 (10?) thousand feet and a not very humid at all 90 degrees.

For those watching at home, Rampart definitely qualifies as “trail riding,” and I saw a few other 426s up there on the trails in addition to our group’s five (yep, 5) ’01 426s.

We did not ride down in the Denver area, however.

I believe mikeolichney runs a 38 pilot (and 155 main?) in his ’01 and he lives in the Denver area. That, my 01 at Rampart, and Mr. Dean’s advice should get you close enough for the bike to run okay. Okay as in you don’t spend too much time thinking that something must be wrong with it.

Point being if your bike is running that bad you either:

1) have an incompetent mechanic (or entire dealership) trying to jet your bike.

2) have something else wrong with it.

So don’t give up, life is too short to ride a poorly running YZF. You may try getting service managers from various other Yamaha dealers on the phone, explaining your predicament, and picking the most intelligent answer. Whatever you had to pay to CO Powersports to (not) fix your bike you should demand refunded in full, if only for the opportunity to vent a little.

Sorry, stories like this just make me mad.

Hope this helps :)

Dodger, these tips are from a wide survey of riders from around the world. Your dealers mechanics may not have seen all the variety of problems we have here in the past 1 1/2 years. This web site was created because of problems like yours. Try the settings, they are your best course of action.


OMG, the dealer is BSing you bad! My '00 WR runs great from 400-2300 feet with just a slight tweak on the fuel screw. Trail or MX.

Friends with both '00 YZ and '01 YZs have absolutely no problems with trail riding--that's all they do! Both of them ride in Ohio, Indiana, Vermont, and WVA with drastic elevation changes w/o problems!

Writing letters is fine, but when I'm treated poorly at my dealer, I just stand there and raise my voice about 4 notches to ensure all the customers in the shop hear me complain.

I only do this when I'm treated poorly or brushed off. Thankfully it's been awhile since I've had to do this.

Maybe say loudly something like:

"So you mean to tell me that Yamaha will not support problems if I use this dirt bike on the trails? If I have the same problem on an MX track, will you help me then? I just don't see a difference between riding a track at xxxfeet or riding a trail at the same altitude"

Do it on a Saturday afternoon when they are busy. Do NOT go into the office with them, state you're issues right on the showroom floor!

You didn't state what your problem really is, so I can't help with advice on how to proceed.

The CDI and carb is a good place to start, kudos to MikeO for trying to help you personally.

With the exception of the aircut valve, the WR has the same carb (albeit different settings but YZ settings work great on a WR w/ YZ timing) as the YZ.

That boils my blood, man. Yamaha's customer service needs major improvement!

I have a 99' 400 that I ride a lot on mountain trails (Utah) in the summer (desert is to hot and dusty). I go from 4000 ft. to 11,000 ft. on some rides with absolutly no problems. I can't notice any change in performance. If anything these babies are less sensitive than two-strokes.

I run an E-series exhaust with the stock header.


178 Main (one larger than stock)

48 pilot (one larger than stock)

needle in 5th position (one groove richer than stock)

No hesitation. No fouling. Easy starting.



99'YZ-400 (Mine)

00' TT-R 125 (Hers)

91' KX-125 (Son #1)

93' CR-80 (Just sold it. Looking for another TTR125 for Son #2)

99' PW-80 (Son #3)

If I'm not mistaken the DRZ offroad models share the same carb as the Yamys and that is a TRAIL bike.

The KTM four strokes use the same carb as well (including the enduro models).

Some Yamaha dealers will do anything to try and weasel out of their responsibility for any repair. When I asked about '00 clutch problems, it was, "duuuh, we've sold at least hundred of those things, and you're the only one that has had any problems of any kind, and duuuuh, and if you really knew what you were doing, you would remove the plates between each moto/ride and soak 'em in oil, just like the pros, duuuuh, boy, we have some dumb customers".


Holeshot's Page

[This message has been edited by holeshot (edited 08-12-2001).]

I'm John, bought a used YZ426 from Fay Meyers last month, took it to Colorado Power Sports (Castle Rock) to have it jetted. Never complied about it, exempt to a friend at work. Don't know Mike, Mike’s friend XXXX XXXX or Dodger. What a small world

The bike ran like crap after CoPo (Castle Rock) missed with it. I used James suggestions and now running:

#40 pilot jet

4 1/4 turns <----- See below

Stock needle clip #3

#155 main jet

Lowered float level 1mm below stock (7mm) stock is (8mm)

Don't know where the pump timing screw is so didn't mess, gas doesn’t hit the slide when opening the throttle.

Cleaned the carb meticulously, used carb cleaner and compressed air.

CDI magneto ohmed fine

Primary coil ohmed at twice the resistance (part on order)

Valves and timming is correct.

Bike runs great at top end but, still runs like crap at bottom end. Sputters from idle to 1/3 throttle. Pops allot when getting off the gas.

Obviously the pilot screw it turned out much farther than normal. If I turn the screw out 3 to 4 turns, with-in 30 seconds of starting the bike the exhaust pipe is glowing bright red, give it the throttle and release the engine idles high shoots an occasional flame and returns to normal (sign of the lean setting). 4 ¼ is were CoPo Castle Rock left it also.

Any Suggestions



I take it your accelerator pump DOES work then? It should shoot a stream of gas out the front of the carb for at least a few seconds.

If you leave the fuel screw like that it may fall out, anybody who would attempt to set it that far out is a moron, do not let them touch your bike again. I doubt it is even working properly at that setting. I also doubt the 155 main has any bearing on your problems, but that seems too lean.

The exhaust pipe on all these bikes will glow. That is normal.

Fuel delivered by the accelerator pump will make them all shoot a flame too if you blip the throttle enough.

My advice to you then is to put the fuel screw at 1.75 turns, anything over 2 is too much. If your bike doesn’t run at least marginally correct at this point I’d say you have an electrical problem. In that case what I would do is find a good friend with a two-six (don’t matter what year) and start swapping parts.

The primary coil thing is interesting, I’m not sure what to make of that, I thought they either worked or didn’t. Maybe that is your problem. But put the fuel screw back to 2 turns or less regardless. I promise you it can’t be too lean at 5k feet with a 40 pilot (I run that at 4k feet and 1.5 turns out in my ’01).

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