Fouled plugs = foul mouth!

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.

Things just dont sound right ? I've got 7 months on the SAME plug, and I race the hell out of my bike.


Sounds like a possible Accelerator pump and or CDI problem? Are you sure you got the correct needle in the bike stock?


I know how you feel. See my post on $6k paperweight. I am at 5000 ft in Colorado. My 99 would run a whole year and not need a new plug. My 01 would foul a plug in 20 minutes. Yamaha says there is a gremlin in the carb related to assembly. I get my bike back tomorrow with a brand new carb, will keep you posted.

Last weekend I also fouled out a plug. Not while riding, but by trying to start it. I swear that thing is just dumping gas when I'm kicking it. No, I'm not touching the throttle.

This may be premature because I've only had the bike a week, but I think I have that same problem with my '01 250F. Super rich. There could be a problem with the accelerator pump. Moving the clip did nothing but make it cough, pilot screw did little as well, I went extreme lean and saw little change. I have NOT changed jets simply because my dealer was out of stock of the most commonly used. So I'll wait 'till I can play with each circuits jets, before I condemn the flat CR. Becuase I have little experience with four strokes I have to believe it's my learning curve tuning it. Maybe ridiing style?

I'll know after this weekend


Duece, don't blame yourself, its not your riding style. This whole thing about fouling the bike by wicking open the throttle too fast is a cop-out. There wasn't anything you could do to my 99 to foul a plug. Ten or fifteen people must have rode that bike with no problems. I changed the plug once in over two years, because I was shimming the valves and it was right there. It didn't need it. Thats the way it should be. The 01s are clearly more fussy, even more so than the 00s. One day last month as I pushed my fouled out bike into the pits I looked around. There were two guys relentlessly kicking theirs, like a panicked surgeon defibrillating a long-dead patient. There was another guy with his tank off changing a plug. I believe its worse here at altitude, but Yamaha clearly has some issues to deal with.

I have fouled only once on mie, but I have a friend who said his would foul a plug EVERY time the throttle was blipped when not running, the next time he started it...


Except for the apparent problems with th '01s, I think it's mostly cold starting technique that fouls plugs. When I first got mine, I would blip the throttle when it started to keep it running. This fouled the plug every time. Now, I pull the choke on, start it, let it die, start it, let it die, and so on, till I can let it idle with the choke off. Even then, I let it run off choke for a minute to be sure it's sufficiently warmed. In my (short) experience, if you have to blip the throttle at all, it just isn't warm enough.


i've come to the conlusion that both the motocrosser & the enduro machine are over carbed. they dont need a 39mm carb.

i read a few yanky mags in the summer & the bikes were tuned; but no tips were being given away. bigger engines, yup. bigger exhaust's yup. bigger carb; er um!

the magazine guys presumably had a good look around the bikes. the CC's had gone up on some but the size of the carb hadn't had it!

i'm sure you can run a huge carb clean just as you can a little 'un. but it is SO much harder from zero to quarter throttle to get it right.

my old pantah road racer produced 65bhp at the rear wheel which is probably 35 bhp from a 300cc cylinder & 47 from a 400cc cylinder.

i used to swap regularly between mikuni flat slide 36mm carbs & dirty great 41mm malossi del orto's. the difference? 1 1/2 bhp!!!


I have not had that problem yet on my 01426 but it is still fairly new. I live in CO. and was even able to crank it up in the 6 degree weather. third kick fired right up. Does this problem start as the bike gets older or more used?

my problem had nothing to do with starting. It would start first kick, every time with a fresh plug. After five minutes, it was backfiring like crazy. At ten minutes, it wouldn't idle. At 20 minutes, it was lurching and missing, then dead as soon as the revs dropped. Mine now has a new carb compliments of Yamaha, but we now have snow on the ground, so I can't test it.

hey MikeO where do you ride at in CO. Only place I have been is Burthoud(?). New to the Ft. Collins area was wondering where some other tracks are at.

Hey, I love Berthod. I race the VDR hare scrambles series there. The races are 2 1/2 hours plus a lap. The third one is this weekend, if the snow melts. VDR does a great job with that series, but Berthod turns to grease when its wet. You probably found that out. Don't waste your time with Thunder Valley. Watkins is also good, better dirt, but I find the two 90 foot tabletops pretty scary. I can't clear them yet, they are twice as big as any jump at Berthod. Also there is a track at Miliken, which is close to you. Its OK, very tight, jumps are sort of weird. But I haven't been there this year. It also has limited hours, while Berthod is open 365 days a year. Berthod's my favorite. I am an old guy, I race the senior class. Well, this year I start the race, anyway. My plug fouls and then I DNF. I wish I had kept my 99. It sounds like your bike runs good, wish mine did. I have a red 97 F150, good hearing from you. Oh, also check out Rampart range in the summer. Tons of berms through the woods.


The only thing that fouls a plug is the introduction of more fuel than the plug can burn off of itself during the combustion process.

Running a hotter plug is not the fix.

Take a look in the manual and you'll see that each jet (or needle and slide) affects the mixture at different throttle positions (aka RPMs).

When you re-jet a carb for a change in altitude, you'll need to re-adjust the entire range of carb operation, (fuel to air delivery). Just changing the main jet and turning the needle will not suffice.

When you decreased the main jet you pro-longed the life of the plug via higher RPMs. Now you need to go back and fix the low range.

Also, most people don't operate these bikes at the altitudes that some of you are riding at, so don't expect that the jets you received with the bike are suitable.

Hope this helps.


hey mikeO, if i can't get things to work out, what is the procedured for hitting yama up for a new carb?

DaveJ, there is something else going on with some of these bikes. I know it sounds crazy. Two dealers tried to fix mine, going to what they considered ridiculously lean jetting: two sizes down on pilot (and one up too), three on main, leaner needle on the leanest clip, all fuel screw settings, an adjustable stop screw on the accelerator pump to limit fuel squirt, and it still fouled plugs. Like crazy. It was worse when it was cold. The dealer is telling me it is a faulty seal in the carb. Yamaha gave me a new carb (they are $1200!) OUCH if you need to buy one. So I am guessing fuel is somehow leaking into the venturi though some path other than the normal fuel circuits. Maybe the cold weather warps the seal open more due to thermal expansion. I didn't take the carb apart, when you buy a new bike, it better run. Thats Yamaha's responsibility. My 99 ran stock sea level jetting with just a minor tweak of the fuel screw. Even ran OK at 10000 ft elevation.

A good friend of mine, who is a very experienced racer, had the exact same symptoms with his 00. The first dealer I took mine to (Colorado Powersports) tried to jet his for months! Every week end he went to the track, fouled a plug after 5 minutes, and then took it back. After a few months he swore he would never own another Yamaha. He made them give him a YZ250. He took it down to the Honda dealer and traded it for a CR. I hope that's not going to be me, this is my 6th Yamaha. There is an issue on some of these bikes outside of normal jetting tuning. Its funny because most of them run great up here with a 158 main and maybe a 45 pilot. It may turn out to be the new accel pump mapping, but Yamaha has not been able to sort it out with my bike.

To Timdawg: COMPLAIN! Be a major whiner! Don't give up! Make a fuss like I have all over this forum! Wanna bet Yamaha personnel read this site? Keep it up until they fix it. You paid alot of money for a new bike, it had better run. Be careful about doing stuff yourself, I have seen manufacturers blame the customer if possible. Make the dealer you bought it from sort it out. If you bought it used, well, that might be different. There are alot of people on this forum that may be able to give you guidance, but if its the same problem I have, you may be in for a hassle. Best of luck, don't give up.

So Mike,

Does this mean you still have a problem with the new carb?

Well, it looks like I've found a group that's seeing the same problem I've been wrestling with for the past month. I bought an '01 426 in mid November and have been fouling plugs: 1 for every 2 hours of riding. I've owned a '99 400 and a '00 426 and never fouled a plug (this is at 5000' in Albuquerque, NM). I have tried the usual jetting changes: 38 pilot, leaner needle clip and main. The bike ran poorly with leaner clip and main so I actually went richer but left the 38 pilot. Still fouled plugs. Next I took it in to the dealer and their mechanic put on an EGA using the stock jetting (he couldn't get it to idle with the lean pilot jet). The EGA said everything was fine: no excess hydrocarbons, good CO and CO2 (so where's the soot/excess fuel coming from?). I haven't ridden the bike since I got it back on Tuesday but it's hard for me to believe that it will suddenly stop fouling plugs for no reason.

I've tried various starting regimes/procedures with no effect and the other bikes had no problem whether I gave the throttle a couple of squirts to aid starting when cold or not. It sounds like there is clearly a carb problem with some of the bikes. I'll try riding again tomorrow and see of the stock jetting has miraculously cured the problem.

Most of the people with problems seem to be at altitude (which causes the mixture to be richer) but come summer I'll bet more will see this problem.

This is such a curious problem. How can some bikes be fine while others of the same model have these recurring plug fouling syndromes? Like you, my 2000 YZ 426 has no plug problems. I'm on my 2nd plug and have had the bike since March 12th. I wonder if some carbs have an abnormally high float bowl level causing a flooding into the pilot circuit. As a test, why not set the float level a little below what is called for in the manual?

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