Fouled plugs = foul mouth!

This saga is like the 'Neverending Story'. Maybe reading this thread infects your bike...

But really, I've already posted that my '01 suffers the same problem of fouling plugs. Jetting changes have no effect.

It's NOT the air filter. Yes, I'm familiar with the problem of trying to start the bike with a freshly-oiled filter but I've use thrououghly dried filters and the problem persists. These are the same filters I used on my '00 426 with no problems.

I've been more observant about the bikes behavior and have noticed a few things:

1) the bike starts fine (new plug), idles great and initially runs cleanly,

2) after, say, 5-10 minutes of running (varies depending on temperature) the bike goes through a phase that involves missing/backfiring/popping and the idle speed drops so that it's necessary to constantly blip the throttle in turns,

3) after another ~10 minutes (if the plug doesn't foul) the bike starts to run better as if the mixture has leaned out to normal levels,

4) the bike will now run properly (or almost considering the almost-fouled plug) for the rest of the day - unless you stop and take a break,

5) after the break, upon restarting the bike it will go through the same period of poor running/popping/backfiring and poor idle and then clear up again (provided the plug still works).

I thought that the answer might be the temperature/temperature distribution of the carb due to a misassembly/machining error but now, after reading about how a replacement carb started suffering the same problem after everything was fine for a while, I don't know. Also, I seem to remember that the first ride (during break in) didn't go through the poor running phase.

But if it's not the carb then how did a replacement 'cure' the problem for time? This is getting very strange. In my garage I have my '00 426 which has always run fine. I'm wondering if I should try swapping carbs/cdis between bikes. I know there have been a number of changes including spark advance so I don't know if I'll learn anything.

try giving that carb the bst blow job it's ever had. a bike runs clean for weeks & then lets you down & all you've ever done is oiled the filter.

i think rob c is very close. now i've had the problem & cured it (unfortunately i did 4 jobs at once so i'm not sure)i'm convinced tha some passages are very susceptable to muck.

i found the pillbox slit grimy, it shouldn't be grimy!


I wanted to mention some things that may be important about debugging this fouling problem, and the carb swap my bike had. The HS I did on Dec 17 has a 15 minute, 9 am practice lap and the bike ran great, no warm up issues. I know it was below freezing because a friend of mine frostbit his fingers and I lent him better gloves for the race. I did not look at the plug or do anything to the bike after the practice lap, other than completely fill the tank. Then at 11 am I raced for 2 1/2 hour HS, and the bike ran perfectly for every minute of the race. By then the temperature was 35 to 40 F. After the race I pulled the plug. It was jet black, so I swapped it. There was also a bike wash, an oil change, an air filter change, and the stock tank went back on to replace the HS tank. The next week was busy for me, my daughter was born on Thursday. On Sunday, Christmas eve, I tried to ride again. The temperature was about 35 to 40 F. The plug fouling problem had come back with a vengeance. I couldn't get more than 10 minutes out of a plug.

why are we running plugs anywhere near black?

any other transport would have a tan coloured plug wouldn't it? it would be nice to talk to a plug guru.

i bet the top gp lads don't have this problem, i bet they had a hush-hush circular come round & they've been laughing at us ever since.

has anyone had all this set up on the rolling road/dyno?

i run a 45 pilot on my WR here in the UK & i 'only' have a 162 main jet. i've backed the acc PJ off as much as i dare.

when roadracers start to run out of battery power on their total loss ignition bikes, the plugs start to foul!

just some idea's.


Just an update on my 01 426 with the swapped carb. Its back at the dealer now. I gave the mechanic a copy of all the related posts from this forum (thanks everyone, this helps with credibility when he deals with Yamaha). Yamaha is telling him that similar problems at altitude have been caused by too high a float level (as suggested by seveal people in this forum). The dealer checked again, adjusted it as low as possible. No dice, black plugs still. I am still pushing for an ignition swap but they don't have a 426 at the dealership right now. I will keep you posted.

Mark me down as a chronic plug fouler as well. I'm going to have to start reading the YZ426 forum on a more regular basis! I have spent more time swapping out fouled plugs and leaning out jetting then I care to admit on my 2000 WR400. My plugs do not come out wet but rather sooty black. I've measured jets, practiced good warm-up techniques and everthing else I could think of or read about and still foul plugs. There was a post on the WR400 board where a reader reported that a CDI corrected his fouling problem. Hopefully this problem will be resolved.

Hey all, just another note from a

plug-fouling Y2K.1 YZ426 in Colorado.

Great info and I'll go get registered as

soon as I post this.. but to the point;

Mine's doing the same thing except it

runs fine all day on the same plug. Will

NOT start the next day on the same plug.

I'd hoped jetting was the answer but right

now I'm being skeptical. I'll contact the

dealer tomorrow (Apex, Colo Spgs) to see

if they've gotten any updates. Thanks to

all for the posts.

don't know if this is your case, but maybe it'll help. i own a 99 r1 and an 01 426 and if you "lug" the engine too much they will load up, ex-if i ride the r1 around town endlessly w/o taking it out on the highway and blasting it open, it starts to run like s@#*, the difference w/ the 426 is it doesn't have 3 other cylinders to help it keep firing (not to mention an elec start). what i have found w/mine is if i trail ride all day i NEED to open it up somewhere to clean it out. doing this, i have had no fouling problems, hope this helps

TMX AND MY DOGS BITE!!!!!!!!!!!!!!


Thats not whats happening to my 426. My plug is black in three minutes, and I am doing all my testing at Berthoud, which is a wide open mostly third gear track. There is no lugging. My dealer disabled the accel pump yesterday to see if it still gives black plugs. That wasn't it, plugs are still black. My money is now on the ignition.

yep but is it

the ignition

the generator output

the coil output

this is the first bike i've ridden that backfires on a closed throttle. my only other experience of this is with bikes that had leaky exhaust manifolds.

but the backfiring isn't leaky exhaust. is it ignition timing on the closed throttle?

both the wr & yz suffer so i think the generator output can be dismissed.

so carb, black box, coil & lead. that's it.


This is a great thread - The game's afoot! Yeah, this is definitely a weird one. My shop has ordered some accel pump parts to try and see if that may be the cause but the more I ride the bike the more I'm convinced that it's something else. After I ride for a few minutes the bike makes a brief stop in the twilight zone: popping/backfiring and no idle for a few minutes (long enough to screw up the plug). If the plug doesn't foul then I can ride for the rest of the day -slow, fast, throttle/no throttle - with no problems at all. This couldn't be the accel pump which would dump too much gas each time I accelerated. If I stop for a few minutes for a break and then restart the bike will do the same thing. Something is temperature related: CDI/carb/magneto(?). I've been putting off doing the swap-thing with my other ('00) 426 as it's for sale but I just might start. I've also thought of using a hot air gun (on low) to heat various parts to see if I can generate some response.

The dealer is still working with Yamaha on my bike. Despite disabling the accel pump and still getting black plugs, they think that it is the accel pump. They insist it is not the ignition. They are also saying that the stock position of the carb in the boots block air passageways and they get a little better results by rotating the carb in the boot (Yeah, right and I have some nice waterfront property in the everglades for sale). Anyway, I am going to pick up the bike now and try it. It is 60 degrees out and I have never ridden it in such warm weather, so that will at least be interesting.

My question to you guys is:

Do any of you get a nice brown plug on your 426? If so, what is your elevation and jetting? I have yet to talk to anyone who gets a brown plug at elevation.

Here is a post I found to be very interesting that came from the 'WR' board from Jamie Williams-

"A friend of mine just bought a brand new wr400 99 model ex showroom stock and it run fine for two weeks and then started fouling plugs every time he rode it.I gave him a hand with his jetting because his bike is the same as mine and we ride in the same areas and my bike is o.k .Rejetting did not help the problem, then someone said what about the C.D.I so he had the bike shop check it with a multi meter and it checked out fine. Still not happy he swapped the C.D.I with another bike and the bike run great so he had it replace under warranty and his bike hasn't had a problem fouling plugs since.

I never thought this would have been the problem but I rode the bike just before and just after the C.D.I was changed and what a difference.

Not an expert just my 2 cents worth."

Definitely food for thought...

P.S. bibainb my bike does exactly as you describe above.

Well, I have an ’00, but I just looked at a fairly new plug and it is tan on the edge of the porcelain away from the electrode. The threads are black. To be honest I don’t pay too much attention to what my plugs look like, but I have never had the problems you are having.

4,500 ft. +, moderate temps (So. NM)




I think your dealer is either an idiot or is convinced that you are. From what I have read of your posts I’d bet the ranch it ain’t your acc. pump. I’d bet the north forty it ain’t your carb at all.

The air passages they are talking about must be the air jets on the back of the carb, I think I mentioned that it sounded like yours (or was it the guy in Albuquerque?) might be clogged. Rotating the carb is silly, the air filter joint, which covers the air jets, moves with the carb and the passages in the joint sit inside the airbox boot regardless of their relative (boot & joint) position. The passage in the joint that feeds these jets is relatively large, I don’t see how it could get totally pinched off (and that is what it would take).

It must be ignition related. I don’t have your patience, I’d have tried the CDI swap already and to hell with the dealer.

Keep the updates coming, I wanna know how this thing turns out (probably with your dealer looking foolish and hopefully with you riding a well-running bike). When they discover how stupid they have been, don’t rub it in (not that I wouldn’t). Be gracious and they may reward you for your trouble (and to help get over their embarrassment).

BTW, totally different bike, but I had a ’95 KX 250 that fouled a lot of plugs, despite a ridiculously lean pilot jet. I swapped the CDI from a friend’s bike at the suggestion of another friend and that was that. I coughed up the $ for the new CDI, another friend still has this bike and it still hasn’t fouled any plugs. I never figured out why it was doing it to begin with, all I know is it was obviously the CDI.

I believe you mentioned that your bike ran perfect at times but continued its problems at other times. Supposing the CDI is the culprit, whatever it is doing it is intermittent enough that your dealer hasn’t encountered the problem when testing it.

Maybe you should coyly suggest this possibility to them in the hopes that they will allow a swap. At this point they should do whatever you say, IMO.

[This message has been edited by Hick (edited 01-04-2001).]

i read the WR post & borrowed my friends CDI. bike really wanted to start for the first time. but the plug just couldn't cope. wire brushed it & haven't looked back since!

nobody has picked up on this low generator/coil output (don't forget it still leaves the lead & cap!) causing the sootiness. as i said this is how roadracers run just before they've had it. although to be fair they misfire at top revs first.



I have a stock 01 426. Ive fouled the plug once during start-up( I gave the bike a little too much throttle right after the bike turned over and it died. After about 20 more attempts to start it, I checked the plug and it was covered with carbon and wet). I sand blasted the plug, put it back in the bike and it fired up on the 1st attempt. Other then that one time, the bike runs great and the plug is usually black, but not carbon covered. Its in the 20's here, sea level, and I havent messed with the carb at all. Sorry to hear about all your troubles. Is there any kind of lemon law that might cover you? In New Jersey if a motor vehicle dealer cant fix the same problem in (3) attempts you can get your money back or another new vehicle-maybe a dirtbike would be covered by the same law. If I had an unresolved problem with my bike, I would get a replacement or the dealer would mysteriously burn to the ground. Thats a bunch of **** -for 6000.00 a bike should run for more than a few hours! Good luck.


I rode my 426 yesterday and it ran great. All that was done was a slight rotation of the carb in the boot and it had 114 octane leaded race gas, and it was 60 degrees out. I think it is the temperature that made it run well. I bet as soon as it gets cold, it will go back to plug fouling ways.

Whoa! The circus continues! Here's another piece of the puzzle.

Today, in Albuquerque, NM, it was 54 F (the warmest I've ridden in) and I took the '01 out for a spin. It ran the best it has since I picked it up on 11/11 (that, BTW, is the start if Fasching - a german holiday that runs until February or some such and, no, that is not short for fascist it's more like Halloween but they actually charge money to watch the small parades that every town puts on). Sorry, way OT. Anyway, the bike ran better at 54F than at colder temperatures with only a much reduced period of running 'fat'.

The CDI eh? I almost swapped the '00 to the '01 but as I stood there, ratchet in hand, I couldn't think of a single reason as to why that could be the case. The problem is intermittent, seems to be temperature dependent and when I tried applying a hot air gun (not too hot!) on both the carb and CDI I couldn't get any response. Still, based on some of the other remarks in this thread I may have to give it a try.

Hey guys, the latest straw yanked from the bundle here in Colo Spgs is that the TPS is faulty and therefore will not allow enough spark. (Kinda follows the CDI theory) I don't have *any* details at this time but as soon as I find something out, I'll forward it here.

Please blbainb if you have access to another CDI you would be doing a huge service to your fellow plug fouling YZF/WRF riders. I agree with the problem being intermittant but only on the fouling part...the root cause could be constant and other variables such as ambient temp, engine temp, jetting, humidity, altitude, throttle usage, etc determine if your plug survives or even goes into the coughing, sputtering and backfiring and ultimately fouled plug mode. I've got 1100 miles on my bike and always get a black sooty plug despite lean jetting. This problem will not go away in warmer temps.

I will be a happy camper when someone figures this problem out.

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