Plug Fouling

I just bought a new 2002 426. I love the bike. Ok, here is the downside, I keep fouling plugs. I run stock jetting and run 93 octane gas. It is like, it gives me no warning. It can run fine and the next time you try to start it, it just want start until you change plugs. Please let me know if of you have had this problem.

Your problem could a couple of things. First, make sure you are not twisting the throttle when kicking it over. That will cause the accel pump to squirt gas into the engine and flood it. Second, make sure your air filter is clean as these bike suck A LOT of air. Third, make sure you turn your gas off when not riding.

Hope this helps you, Ernie

I always keep my air filter clean and I don't turn the gas when kicking it. This thing really has me baffled.

Check your fuel screw position. Turn it in until it lightly hits the seat. Back it out 1/4 turn at a time and see if that makes a difference.

Does it start easily when the hot start is pulled?? If so, you may be rich on the pilot.

Have you had the carb apart recently?? If so, check to make sure everything got put back together correctly. There have been posts recently about putting the carb back together wrong.


I have only had the bike about 2 months, and have never had the carb. off. I turned the screw in, and then turned it back out one full turn. The stock setting was sopposed to be 1 3/8 turn, but when I turned it in, it turned in about 2 1/2 turns. Do you think this could have been the problem?

Very possibly has an effect on the fouling. Was the bike new when you got it or was it used?? If used, check the jetting to see if it in fact stock. Sounds like you are running too rich. Try it with the fuel screw where you set it and if it is better you'll know that you are on the right track.


DUH, just reread your first post. Sounds like whoever set the bike up did not check the fuel screw. I think you'll find that it will run much better with the fuel screw turned in to where you have it now.

I would guess that Kansas is pretty flat. What elevation are you at?? One other thing to consider...the stock air filters do not allow great air flow. I use either No Toil, Twin Air or White Brothers filters. They are single stage which allows better flow but still dense enough to catch the dirt and dust.


FWIW, my '01 has done this since day one in cold weather.

Yes, mine is jetted correctly. No, I don't twist the wick when starting.

It'll run great all day on a new plug. When I try to start it the next

day, it will foul. A friend bought his in the warmer months of

last year and ragged on me all summer about how his still had the

"factory installed" plug in it. As soon as the weather turned a little

cool, he started fouling plugs.

It appears to be a cold weather, high altitiude phenom for us. FWIW,

we're all competent mechanics and jetters.


Well, i'd have to say the same... but I have remedied it! If you adjust your air-screw (fuel screw) daily to find the best setting off idle you should have good luck. I was recently (one week ago) riding in 20-40 F weather with some friends in Idaho. I didi this for three days in a row, no problems AFTER I got the screw turned out a little leaner. I think they are a bit rich stock, and for those of use up high, and in the cold, this should help. I have also heard that the BK carb mod fixes this problem. I am going to do that this winter and let ya'll know how it works during the cold spring riding that I look forward to every year.....

I bought the bike brand new, so I know it has the stock jetting in it. I also run a twinair filter in my bike, so I also know that it is getting enough air. Does anyone know anyone who has done the BK mod on a 02 and likes it. I have heard that it works really well on the 00 and 01, but most who have tried it on the 02 have had alot of problems with it. I am running my air screw one turn out.

Hey Candlkirk,

I had the exact same problem as you to a tee. :D

I fixed mine by simply dropping the needle down one and turning the screw 1/2 turn in from factory...I haven't foulded a plug since.. :D ...

except when my kid pumped the ol throttle 30 or so times when I wasn't looking :)

I'm going to do the Bk mod myself.....wish me luck.

I had this same problem with my 01YZ426, and so did alot of other guys last winter (also seemed like high-altitude guys had it worse). With a fresh plug, it would go about 15 minutes, backfiring and sputtering more and more until it died. Then it would not restart without a new plug. It was outside temperature related, below 40 degrees it would do this. IT WAS NOT JETTING! The plugs always looked sooty black (unlike my WR250F had a tan insulator with the same CR8E plug). I jetted the bike ridiculously lean and it still fouled plugs in cold weather. Besides, colder weather makes the air denser and therefore the jetting leaner. I hated the bike. I DNFed a couple of my HS races and ruined my season. It was going to be my last Yamaha. A new CDI "fixed" it, but now I do not think that was the problem. By then the weather had started to warm up. It ran great all summer. I also did the BK mod, which I thought would help. Then for our first HS of this season, in late October, I got halfway through the practice lap (about 6 minutes) and it did it again. It was cold, about 10AM in the morning. A four-wheeler towed me back to the pits (how humiliating!) and I put in a new plug. By noon race time, it was 50F out and the bike ran fine. I got 5th out of 26, which was good for me considering several of the four guys ahead of me were A riders. I was happy about the race but worried about the next one in November, which would surely be colder.

I did some reading about spark plugs and their heat ranges. One article said the plugs need to be in the proper heat range to burn off the carbon on the insulator, but not so hot as to melt things. So I tried a CR7EK plug on the bike. I thought maybe the colder outside temps make the engine run cooler, and the plug cant burn off the carbon. The next cold day it ran great! I looked at the plug, it looked totally different on the ceramic than a CR8, it was tan and white rather than the sooty black. Same jetting, different looking plug, so forget looking at the plug to jet. I tried a CR8EVX yesterday for grins, and fouled a plug! I just ordered a four CR7EIX plugs from for $6.40 each ($15 at my dealer). IF YOU HAVENT TRIED A CR7 FOR THIS PROBLEM, YOU NEED TO!

mikeolichney, I forgot to say that I am runing a 7 not a 8 plug. I have tried both and it does not seem to make a big difference. It fouls both about the same.

MikeO... thanks for the input. My previously-smug buddy and I

were discussing this very thing this very morning. He is going to

try to CR7E route as well. FWIW, *his* bike came with a CR9E

in it..

I went to Texas about two weeks ago to race and fouled a plug

while there, but was totally my fault. I hadn't given the bike time

to warm up and blipped the wick and it went <blurp.....> and I was

pulling the tank... I haven't had a chance to rejet and ride since

I got back. Just for the record, I was 1st VetB and 13thOA B.

candlkirk, I am bummed. I know firsthand how frustrating this problem is for you. The plug is clearly my problem, and I was hoping that was it for you as well. Mine starts fine but then goes bad after 5-15 minutes of riding, but you say yours is a starting problem. A good long warmup seems to help mine some, but the plug was key. If the tips above don't help, you may have to try parts swapping with a friends YZ426. Try the CDI, coil, carb. All those parts swap easily (believe me I know from last winter).

Also, you know about oiling the airfilter, right? Be sure you let it sit a day or so before starting it. These bikes pull alot of vacuum and some people have sucked that thick filter oil into the carb passages. Also, I have a kouba fuel screw adjuster and I can reach down with my gloves on and adjust it in a second. I always try to make mine backfire just the smallest amount, this way it starts without the hotstart, which is good for the dead engine starts that we have in our HS races.

Merfman, nice job on your race! Let me know if the plug helps, I am so psyched that I finally have my fouling problem solved. Was the race you did the RMEC Texas race on 11/18? Do you know Eric Croy, he is another top B-rider?

[ December 04, 2001: Message edited by: mikeolichney ]


Yea, it was Boondocker's that I went to... as far as Eric, I'm

not sure.. his name is familiar but I was chairman of the RMEC

for 2 years so I have a lot of names that sound familiar. I'm

semi-retired from racing all the RMEC events so I don't go to

as many as I used to. I think I missed a total of 4 events in

12 years so I just kinda burned out.

I go to Texas most often 'cause I love the 20% HP boost down

there and when the conditions are right, it's hard to beat racing

in that Texas loam. Boondocker's was definitely the best race

I've ridden this year, both the conditions and layout were very

much to my liking.

If you ever get down COS way, drop me a line. That goes for

any of you reading this BTW. ...


First you should take the 7 plug out and put the 8 back in. The bikes run hot enough as it is. I had the same problem. I just rased the needle setting in the carb one notch and left the jetting stock.

I had a ton of problems when mine was brand new. Twim Peaks Yamaha in Longmont replaced the whole fule system, CDI, stator,coil, carburator, plug cap. and that finaly fixed the problem. I also run race gas, 112 octane. I havn't fouled one since. Try running the NGK platnum plugs. What does your plug look like?

Originally posted by mikeolichney:

Also, I have a kouba fuel screw adjuster and I can reach down with my gloves on and adjust it in a second. I always try to make mine backfire just the smallest amount, this way it starts without the hotstart, which is good for the dead engine starts that we have in our HS races.

Do you know Eric Croy, he is another top B-rider?

[ December 04, 2001: Message edited by: mikeolichney ]

Would that be the Berthoud HS?

Are you going to be there this weekend?

I know I will! I can't wait, I have not ridden in 5 weeks, since I dislocated my shoulder at the Milliken HS back in Nov.

I think I know Eric, does he work at Powersports on 55th in Boulder?

Hokie, yes I will be at Berthoud for the HS. I haven't missed one in 4 years. Hope weather holds! Look for us in a white/green Shasta Travelmaster towing a enclosed trailer. We park over by the timing gates/whoop section on the north track. Stop by and say hi. Eric works at CPS on 55th, but broke his hand in a (dare I say it?) barfight, so he won't be there. What class do you race? Did you go to Va Tech, trying to figure out your name?

I have had several TT people, most recently Scooter, come up and talk to me because my name is on my chest protector. I guess my lack of imagination pays off.

BTW, although I have not actually fouled a CR7 plug yet, my bike is still exhibiting minor problems (backfiring/stalling). Maybe its the ethanol in the gas here, I am going to try racegas soon. What are you other CO guys running for gas now?

[ December 11, 2001: Message edited by: mikeolichney ]

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