Fouled plugs = foul mouth!

Problems on the YZF seem to be very inconsistent - everyone seems to have a little different problem with the '00 clutch and fixes.

I haven't noticed any stumbling or coughing when opening the throttle quickly on my '00 and I'm still using the original plug (one year). No problems with fouling, either.

Yamaha seems to build no two YZF's alike, just to keep things interesting. :)

Originally posted by holeshot:

Yamaha seems to build no two YZFs alike, just to keep things interesting. :)

That’s an interesting observation Holeshot. Somewhere out there is a poor sap with a YZF that will soon have a broken fourth gear, shattered hub, broken clutch basket, loose main gear, and trashed straight key as soon as it quits fouling plugs and actually runs long enough to fall apart :D

I’ve only fouled one plug, a BR9, at 4,500 ft., in the summer, and it was (probably) my fault. But in Yamaha’s defense don’t they just buy those carbs from Keihin and bolt them on?

DaveJ: There is snow on the ground so I don't know yet if the new carb fixed the problem. Hopefully this weekend.

blbainb: Welcome to the club. I hear what your saying about worse in the summer, less dense air so even richer. But mine is clearly worse the colder it gets. My personal theory is that the thermal expansion of the metal due to temperature makes a seal in the carb leak more when its cold. I say this because the second dealer (TwinPeaks) said that Yamaha suspects a bad seal between the upper and middle carb bodies. I'll try to keep bugging them for an answer, but they may never tell me what was wrong with the original carb.

Boit: Both dealers checked my bikes float in the carb. They are convinced thats not the problem.

Hick: I hope I am not going to be that guy. Also, Yamaha does buy the carbs from Keihin, but I heard the new "2" carb on the 426s is Yamaha's proprietary design. Thats why KTM and Suzuki DRZ's have the old carb like the YZ400. And Yamaha sold me the bike, not Keihin. Yamaha put the whole pacage together, so they are responsible. I am going to look over at the DRZ side of this site and see if those guys are having any problem. A friend has a DRZ and it runs flawlessly up here. Have any of you guys heard of plug fouling problems on DRZs or KTMs? YZ400 seemed good.

my have everyones answer. when i picked my bike up the other day. the mechanic who races was having the same problem. he said that he tried everything and nothing really helped. then he put 87 octane gas in it and never fouled another plug. he says that he runs 93 in the summer.


I really don't know what to make of the carb situation just yet. I don't know if it's me or the bike, or both that's causing a problem. Last weekend I struggled. Went to ride it Saturday, but it never lit. When just the day before it started and I was able to ride it for a good two hours. Now, was it the way I was riding that caused it to foul out the plug, or was it a carb problem? I honestly do not know yet. I say this because today, one week later and with a new plug, she fired right up for a hard day of desert riding. I did not look at the plug. I want to duplicate last weeks scenario to see what happens. If it fires ok tomorrow I'll know. I did change my riding style a bit this week. I shifted the bike much more to keep it rev'd. I do not believe you can lug this bike like an XR or a DRZ. I know my bike will not come back up to speed in low rpm's when I quickly get on the throttle. It coughs. I've learned to be smoother with that throttle. Keep it rev'd like a two stroke and shift it constantly. Now is this a normal thing with the Yamaha high compression four stroke? I don't know. Also at the advice of a race mechanic I dumped 50/50 race/premium in it and it ran considerably crisper.

For the record, the bike is an absolute blast to ride. Bar none the funnest thing I've ever been on. I just hope I don't have the FLATCR virus.


[This message has been edited by Duece Exmachina (edited 12-15-2000).]

I've lugged (5-10 mph) my '00 426 in first gear for hours at a time while riding with my 8 year old daughter (DS80), and experienced no problems with fouling - it's a four stroke for gosh sakes. Similar riding on my previous two stroke bike would cause plug fouling.

Since some '01 owners are not experiencing this problem, I'll take a wild guess and say that some of the carbs have some sort of flaw (or maybe ignition related?). In any case, it would seem that rejetting may be a "bandaid" attempt to fixing a more basic problem.

Yamaha may do their best to weasel out of their 30 warranty, unless you're persistent.

[This message has been edited by holeshot (edited 12-16-2000).]

There is hope for all you guys that are fouling plugs with your YZ426s, regardless of what you do to jet. Mine would not run more than 20 minutes with the original carb, no one could figure out what was wrong. After Yamaha swapped me out a new carb, the bike is awesome! I just finished a 2 1/2 hour hare scrambles and it ran great the whole time. Definately a bum carb as delivered! What a hassle that carb thing was. At least Yamaha came through in the end.


Good for Yamaha!!, and good to hear you are up on 2 wheels again!!

Did they say what was wrong with the carb?

Did they say what was wrong with the carb?[/b]

No word yet, I will ask again on Tuesday.

I took the '01 426 out on Friday after the mechanics had looked over the bike and reinstalled the stock jetting (they said it looked fine on the EGA and ran fine during a test ride). Well, my test ride showed that the plug was still loading up. After a short ride (it was blowing 30+ mph - not really a good day) I took it home and pulled the plug: black, as expected.

I put in a new plug and tried again 2 days later. This time I avoided using the choke at all and, believe it or not, it started in two kicks even though it was only 50 deg. F! Very weird! During the ride the bike's character changed a couple of times: from perfect to lean (popping badly on decel) to rich (felt loaded up/ reduced idle speed) back to normal over the space of an hour or so. Pulling the plug showed it to be black even though it didn't foul.

I showed some of the exerpts from this forum and to the Yamaha shop foreman and he agreed to contact Yamaha about the problem. We'll see what happens.

My 426 always looked great on the EGA for CO and O2 (which is all my shop looked at). Yet it fouled plugs like crazy. Maybe because my shop did not run it under load. What about running it on the dyno while using the EGA? Maybe that will show something.

My 426 always looked great on the EGA for CO and O2 (which is all my shop looked at). Yet it fouled plugs like crazy. Maybe because my shop did not run it under load. What about running it on the dyno while using the EGA? Maybe that will show something.

Well, the mechanic talked with the district Yamaha rep and the suggestions were:

1) the 426 is hard to dial in, especially around 50 F (baloney). Try putting in a 45 pilot jet (stock is 42) then use the EGA to adjust for 4 to 5% CO (this is nuts -richer pilot to reduce plug fouling).

2) it might be the accelerator pump putting out too much gas so next try a leaner diaphram (this sounds more reasonable)

Anyway, I've put in the richer pilot but today's ride didn't show any improvement. The bike is in the shop and will be adjusted on the EGA. The mechanic is perplexed also but the shop doesn't have a dyno or any system to apply a load to the engine so the EGA can only be used at idle. We'll see what happens.

The ’01 pump diaphragm(s) are not yet available.

Montclair Yamaha: “Sorry, they are not in the computer yet, that means that Yamaha isn’t releasing those for retail sales yet.” You’d think the district Yamaha rep. would’ve thought to take the 5 seconds necessary to look that up.

Maybe your dealer (R&S ??) can make a special request or something. Maybe you should have your mech. call mikeolichney’s mech in Denver…

Or maybe you can borrow an FCR off one of their DRZs or RFS KTMs (yeah, right :) ) or a buddy’s 426 (more likely). I for one am interested in what is causing you and mikeolichney so many $@% problems.

Hick: Have you ever had your FRC2 carb apart? I have a new carb in my 426 and it ran great for my 2 1/2 hour hare scramble on Dec 17. The dealer had the following jetting (keep in mind our track is at 5000 ft): 162 main, stock needle in position 3 (from top), 42 pilot and 1 1/2 turns out on fuel screw with a CR8E plug. The bike is stock except for a Ty Davis 3.2 gal tank and a Terrycable remote hotstart system. I am running 91 octane Texaco pump gas. After the race I pulled the plug, it was jet black with a little bit of white around the electrode. Clearly rich but it ran so well, considering the problems I had with the original carb, I wasn't going to mess with a good thing. I put in a fresh plug, put back on the stock tank, changed the oil, and replaced the dirty air filter with a preoiled "No-Toil" oiled filter. I did not start it until Dec 24 when I got back out to the track. It started loading up and backfiring again! It is not as bad, does not completely foul the plug like with the old carb, but it certainly is not running like it was on Dec 17. It won't idle, but it will reluctantly restart. The fuel screw did not help in a range from 3/4 to 3 turns out. The temperature was a little colder, but only 10 degrees or so. I am suspicious of the air filter oil. Is it possible that oil from the filter sucks into the carb and gums something up? Is there an air pasageway that a thick oil could clog? My filter was oiled many weeks in advance, stored in a ziplock bag, and then sat on the bike for a week before starting. It seems like it would have had plenty of time to set up. Are any of you guys having problems like this using "No-Toil" air filter oil? Funny because my 99 never had a problem with the same oil and method. Is the FCR2 different in inside layout?

My dealer is closed until Jan 2 so I rejetted the carb to a 158 main, stock needle in position 2 (from top), 45 pilot and 1 1/2 turns out on fuel screw. (Most people I talked to run that here, and I happened to have those jets). The stock jets and needle were completely clean when I rejetted, not great support for my air filter oil theory. But something has changed since Dec 17th. AAUGGHHH!!!!

I am taking a day off tomorrow to go test it out. If its still rich I will try different fuel screw settings and the 42 pilot (I will see if I can find a 40 as well). I can also move the needle to the top and use a 152 main, etc. I am going to go down until the plug shows lean. I was hoping you or or one of the other guys might have some ideas.

I read that something was changed internally from the FCR to the FCR2, I have no idea what. I think the 2 uses “normal” Keihin pilot jets while the slant used a different kind but I’m not even sure about that. The accelerator pump timing is adjustable with a screw on the 2, the slant is less adjustable.

The jetting you describe above is only a little leaner than mine, I ride at 4,500 + ft. elevation in a little warmer climate. I’ve never liked No Toil because it just seems too gooey and restrictive, but I don’t see how this could be your problem. I routinely start and ride my ’00 with freshly soaked (w/ PJ1) air filters and have never noticed any associated problems.

I’ll be sure to remember your story whenever I’m feeling sorry for myself because this *&%# w/ your bike is not good. The latest installment in this saga leaves me wondering if the carb is to blame since you experienced similar problems with a different carb. That notwithstanding I also doubt this is a jetting problem. I’m beginning to wonder if it couldn’t be an intermittent electrical problem. Maybe you should make your dealer try a CDI swap or persuade one of your YZF buddies to help.

If I had to guess at something carb related I’d say maybe your pilot air jet is getting clogged (the slant also had this jet). BTW if it seems more willing to idle with the hot start on that could also indicate a clogged pilot air jet. This little sucker is right next to the main air jet and is hidden by the “air filter joint” that bolts to the back of the carb. From the back the main is on the right, pilot on the left and they receive air via a passage in the “joint,” so check that out as well.

Other than that all I can suggest is the obvious stuff you’ve already heard: (hot start or other air leak, bad gas, remove the gas tank breather check valve).

Thanks very much for the input, Hick. you are right, the jets have changed from 98-99, they are the same as the two stroke. Your suggestion on the CDI is excellent and I think I have a friend with an 01 that will oblige (hope his bike doesn't catch this virus, which is like AIDS for a YZF). Otherwise, I am going to tear into carb after a CDI swap if I don't have any luck tomorrow with jetting. I will look at that air jet when I pull the tank tomorrow. BTW,the tank vent is history and I am going to buy fresh gas.

The dealer is in for an earful when they reopen.

I wonder if its electrical if it could be caused by washing the bike? Maybe I have water in a connection or in the ignition cover? It had been a week since I washed the bike when the problems resurfaced. Am I reaching?

How can this damn thing run great for 2 1/2 hours and then crap out again? DaveJ, any of you other guys have any suggestions?

Originally posted by timdawg:

Thumpers, all of your help in the past has been proven accurate and comes greatly appreciated. For this, I give you my latest problem. 2001 426. Most of my MXing is at approximately 3000 ft. The bike was fouling factory specified plugs on a regular basis - one plug per one hour of riding. In graduated increments, I ran the pilot screw in 1 turn, raised the needle clip position 2 notches (1 below the top), added a hotter than factory plug and changed the main jet to the smaller of the two additional jets packaged with the new bike. Fouling is less frequent - bike runs better now - but I know things could be better. A hard day of riding still ruins a plug. I long for that 'honey brown' properly fired plug. Any suggestions or comments would be appreciated. Thanks in advance.

I can not belive that none of these posts talks about how sensitive these bikes are to over oiling the air filter. It is not even over oiling that does it. I read in one of the new dirt bike mags that they recomend now that when you oil your filter that you need to let it sit atleast over night for the YZ426. If your bike ran good when new and you did not change anything major, and it is now fouling plugs, you have probably pluged or partially plugged one of the small passages in the Carb. It will probably take two or three tries to get it clean, soak in varsol and use alot of compressed air. Take out the hot start and choke buttons, all the jets and use compressed air. It will come clean. This bike will run great with stock jetting at any altitude. Watch the filter oil, use something thick and let it set up!!

my plug fouled twice due to not waiting long enough for the air filter to "set-up" now i oil it the day before i ride and let it sit until i get ready to ride the next day...then i put it in! The jetting plays a major role... also stock jetting doesnt always work for the rider/weather/altitude/bike setup etc!



I get my kicks on a 2001' YZ426!

Friendswood, TX

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