Hey MikeO - carb swap

Mike - Please proceed to enlighten us on your carb swap. I.E.: Method, $$$$, dealer advisement, and your assement of difficulty. What about the TPS - did it come already installed on carb?

My '00 was the worst in terms of the stumbling condition. I put on a P38, one size larger pilot, and adjusted the accell pump rod - problem solved with many $$$$$ - but needless to say I was/am pissed at Yamaha (considering I have only ridden their bikes my entire life). Resultant, this Winter I will buy an XR for my next trail bike - an addition to my stable.


Boomer, I know how you feel. This was my 6th Yamaha and the first that caused me any problems. My 99 YZ400 was stone-cold reliable, never stranding me in two years of riding. What has happened with the 426? Has its performance made it high strung, or does Yamaha have quality issues? (I bet both). I does seem that more people are having problems at altitude. But my 99 would run well at 10000 ft and it had stock jetting. What gives? Before the 426, I never heard of a four stroke fouling plugs like you see in this forum. Did yours foul plugs, or did you just have the stumble? I feel like you do, reluctant to buy another Yamaha because I don't want to go through this again.

The dealer Twin Peaks Powersports took care of everything with the carb. I don't know if the TPS was on the new carb or not. The dealers number is 800-864-4269, ask for Gib. I think it is better for you to talk to them directly, but if they aren't helpful, let me know and I will ask them for you. They aren't open until Jan 2.

MikeO - thanks for the quick response.

My bike did foul plugs - but not to the extent that I could not get it started - but that was only because I kept changing the plug after each ride. Each time I got back from riding I would check the plug, which revealed that the jetting was in the process of fouling it. So, I'd make a jetting change and go ride the following week, hoping to see some inprovement. Chaparral loved me and my by-the-case plug orders.

You are corrent, Yamaha did not de-bug the jetting before releasing the revised displacement-carb combination. It never made any sense to me. Opening the fuel screw on the bottom to (attempt) rid the stumbling just let in more fuel at idle/low engine speed, which is a recipe for fouling. Only on my bike the fuel screw could not overcome the stumble condition. On bikes that turning out the fuel screw did help, those guys are going to be changing plugs often, else be stranded. Another bad thing about this entire problem is the long term effect on the engine exhaust channel. All this raw/rich fuel going through the system will cause build up in/on the piston and ring, in the exhaust valve side of the engine, and in the hedder/muffler.

I am located near Cincy, OH. Sounds like you are out West? I may give the dealership a call but if they are some distance from me, they may not be able to help. I did have my bike back to the purchasing dealership - they were helpless, and called the Yamaha rep line - and they promptly ignored/denied that there is a problem. I guess that's why they made the 2001 changes - duh??.

None of the mags ever thrashed on the stumble of the '00. I have word direct from a US representative that the mag guys are wined and dined by the manufacturers reps here during the evaluation - that goes for ALL bikes - on and off road. So, once again, we need to keep forums like this open so anyone interetsed can learn from the sharing of info.

By the way - check out DirtBike mag this past month. Dr Dirt trashed on a guy who wrote in asking about the stumbling problem. Dr. Dirt answered and revealed his lack of knowledge of the problem by calling the guy a butcher. Oh, wait, that guy has the same name as me - Dennis L. Beam - hey, that is me!!!!!!!!!

signed, "the butcher"


Boomer, you are too nice a guy. Make yourself a real pain in the ass at the dealer. Talk loud about your problem in the showroom so prospective buyers can hear. My friend had an 00 that fouled plugs despite three months of trips to the dealer. It was his last Yamaha. He made them trade him for a YZ250 instead. Squeaky wheel and all that. He never rode it; he took it to a Honda dealer and traded for a CR250. Why is it that the DRZ and KTM guys, which have a similar FCR carb on their bikes, aren't having these problems? Has the FCR2 got some gremlin in it that shows up on some carbs and not others? Yamaha had better sort it out this year or their sales are going to tank when the CR450 comes out. There is a pretty good chance that my YZ will be traded in for one. I have lost a considerable amount of confidence in Yamaha. I hope you get your YZ sorted out somehow.


I'm with you all the way. I was totally humiliated last Summer when a guy on a DR pulled up beside me (my sweat pouring down my face from kicking), shut his bike off, talked a bit about my bike not starting, then said "this is what I like", thumbed the button, and rode off. Staged, I'm sure.

...either way, nuff said....

Hey Boomer, I have a killer 1999 XR 400 for sale. All hooked up for trail use and/or Enduro. It starts all the time first or second kick. I'm was a liitle worried reading all the starting nightmares about YZ's, but I just bought a '99 in mint shape and I have been starting it every day just to see. So far so good. Serious about the XR for sale. Later.

Boomer: I read your letter in the December issue of Dirt Bike mag. This moron ought to change his name to "Mr. Thinks he knows it all". To make a blanket statement that you are a "butcher" when it comes to using your throttle, after you just described that you rolled it on...not snapped it wide open from idle, is merely a thinly veiled attempt to elevate his intellectual elitism at your expense. I find it reprehensible how he insults the 80 and 60 riders who simply ask questions because they do not yet know. I think it's extremely rude and stupid to repulse the future readers who will provide his income. Isn't that like biting the hand that feeds you? I wonder if he'll print my letter?...Hmmm...

I'm having the fouling problem on my '01 426 but I don't think this is any indication of a problem with the 426 in general. I've owned the '99 400 and '00 426 and never had any jetting problems (or any problem at all except for a bizarre cdi failure on the '00) with either bike. I think that Yamaha, which is constantly upgrading/redesigning the carb each year, simply got a bad batch of carbs from Keihin.

As far as the stumble, I think it is possible to cause any bike to stumble if you yank open the throttle fast enough. You almost never see it with 2 strokes because the motor revs more quickly than a 4 stroke. My experience was that I only saw the problem when I just wacked on the throttle out of a turn instead of rolling it on a bit more slowly.

I'm trying to get Yamaha to replace the carb in my '01 426 but that's a different story. The carb is obviously defective: dumping way too much fuel only during a short (~10 minute)period while the bike is warming up. Otherwise it works properly.

Hey guys - this carb swap topic is pretty interesting.

akg800 - I am looking for an XR400 - but I'd like to have a tame, stock bike. Can you clarify "killer" for me? my e-mail address is -- beamdennis@hotmail.com -- if you have any pics, I can pull up jpeg, but I'm not sure what else. Give me some history on your bike if you can - thanks. Also, I'm near Cincy, OH......

Boit - I explained to "Dr know nothing" that when riding at about 10 mph in (let's say 2nd gear), and rolling the throttle rapidly (not WACKING it) open to the stop, it will not rev. Only he left that part out of my letter making it seem like I was running the bike hard and expecting it to rev to the moon. What a dip!@#$%. The bike needs to be no where near any level of high RPM when doing this. It acts like the carb is somehow actually restricting fuel - well, maybe that's the low range function of the rev limiter built into the electronics - anyways I just "DON'T DO DAT" anymore, and avoid any situations where I need to go wide open throttle without giving the bike 10 minutes notice. Ever watch the pros? When Glover, Bell and their Yamaha Race of Champions buddies competed earlier this year they were wacking those bikes big time. I bought and paid for competition level bike, and that's how I want it to run damn it!!!!!!!

blbainb -- Problems with the '01 carb????? Here we go again!!!!!!! The mags are saying that the '01 carb has none of the (now admitted) '00 carb hesitation (stumbling) problems. You are going to have to experiment with pilot jet sizes and fuel screw settings to get the thing debugged. I thought mine was a lost cause, but all it took was many $$$$$$$$, patience, and many more $$$$$$.

Man I hope MikeO is reading this cause he already swapped carbs.....

Rubber down, rippin ground.....


I just bought my first thumper, a YZ250F, that is not even broken in yet. When I bought it I had one BIG reservation and that was the infamous four-stroke capability to "flame out" when you wack the throttle open hard from an idle. I see that you all struggle with this, somewhat, but my fear is having it "flame out" on the face of an 80 foot table. I mean, some tables have approaches that are way too long and you can just cruise into them and SOMETIMES one can even accidentally select too high a gear and just "chug up the 40 mph face" and do it clean anyway. But if the four stroke bogs (flames out, dies, stall, etc) when you give it throttle in this situation, would you be rewarded with a certain forward flip? Anyway have experience with this kind of terror? (All my moto experience has been on 2 smokers).


I have a 00 YZ426 and i have the same problem as some of you but it sounds like my problem isn't as bad as some of yours. But if this will help any ? Yamaha of Canada has somewhat amitted that there is a problem when i had my dealer call them , and they said by rising the pilot jet for instance to a 45 (summer) or a 48-50 (winter) it should help the burping problem ?? After i intsalled a 48 pilot jet and started it up it still did it for a bit but seemed to get better? i haven't had a chance to take it out on the ice yet but as soon as i do i will tell you if it helped at all ?

Freezing in Canada !

Blbainb and Boomer:

I just got back from my second jetting day at the track and am not having much luck. My new carb is better (with a 158 main/#2 needle clip/42 pilot/1 turn it doesn't ALWAYS foul a plug like the old carb), but it seems to have caught the virus the old carb had. I am preparing a longer post that will have a picture of the plugs I used (every plug is black), and a description of my attempt to jet. Blbainb, after reading your description I am convinced our bikes have the some problem. The blurb about it running poorly for a while, and if it can make it through that stage, then it improves, well that's my bike too. Except I still am not sure about the air filter, my bike ran AWESOME for my 2 1/2 hour hare scrambles, not even any backfiring on decel. The only thing of significance I did was put in a new air filter. Do you notice that yours runs worse at lower temperatures? If I can see my breath, I am in for a crummy day.

The dealer opens on tuesday, I am going to see what the best way is to lean out the accel pump (this was tried without success on my first carb). I am also going to go for the ignition/coil swap to see if that helps.

I must admit guys, this experience has humbled me. I am a mechanical engineer and I thought I could handle anything the bike could mechanically throw my way. To date, I have been very seriously ass whooped.

On a better note, my friend went with me to the track today to ride his YZ250F for the first time. His bike runs and starts perfectly. Such smooth, predictable and consistent power (no cough at all)! I felt immediately at home on his bike. I want one!

mikeolichney: Yes, it sounds like we're experiencing the same problem. Interesting, I'm also a Mechanical Engineer(22 years) and this one has me confused also. I had hoped that your carb-swap would be a complete fix but now I'm having doubts.

I've been carefully monitoring ambient temperature and found some interesting details. The problem seems to be very temperature-sensitive. If I ride on a 40 F day or below I probably won't see a problem unless I take a break after riding for a while (which lets the energy from the hot sections diffuse to the carb or whatever). While riding at low temperatures, convective cooling should keep the carb fairly cool.

At higher temperatures (at least up to ~50 F) the problem becomes more pronounced. It is characterized by a period of poor running that appears after several minutes of riding: popping/backfiring/low(or no) idle, that subsequently clears up. After a break the problem will reappear.

Above 50 F I don't know what ill happen. It's possible that the problem disappears completely above a certain temperature. That might explain why the California-based magazine test riders never saw the problem.

I've tried leaner settings (I'm at 5000') but the bike ran poorly and plug-fouling/carbon buildup occurred anyway. I know it's not the air filter and I'm pretty sure it's not the accelerator pump. Could it be the CDI (black box) or some other component? I do have an '00 426 in the garage and if I don't sell it soon I might try some swapping.

Anyway, Mike (?) you might keep a careful record of the air temperature while you're riding to see how it correlates. Maybe we can track this one down.

I am convinced we have the same problem (well, besides both being mechanical engineers for about 20 years). I am also at 5000 ft with riding temperatures between 30 and 50 degrees (CO). The carb swap did not clear up the problem. I am mystified why the bike ran perfectly for my Dec 17 HS and then reverted to its old ways. I will keep a log of temperature and how it runs. I am also going to see the dealer tomorrow about swapping the electrics. Let's keep in touch and beat this thing. I will email you a photo of my plugs, do yours look the same? What is your current jetting?

Mine are black/sooty. I've had some that had a partially white central electrode (after doing a full throttle run just prior to shutting down) which leads me to believe that the mid/upper range is actually lean. I've tried a 38 pilot but that was too lean and the bike idled very poorly. The stock needle (4/7) and 162 main currently work fairly well but I've also tried 3/7 and 5/7 with 160 and 168 mains. I think the bike actually runs better with a richer setup but I'm leaving it stock for now to avoid confusion for the Yamaha mechanics.

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