Plug Fouling?

My 426 must have a faulty CDI unit. I've been through 10 plugs ... If anyone else went through this problem, could you let me know if Yamaha or the dealer took care of the problem or are we out to dry? My bike is brand new (July 01) and has had this condition since day 1.

Will a new ignition completely solve this problem. Thanks!

This is a question to all ... For those of us that have done the BK mod are any of you getting fouled plugs (just did mine this week so I don't know starts on the first kick NEVER did that before the mod). My carb was pumping gas for almost 3 secs. Before I did the mod I was fouling plugs on a pretty regular basis.

I have read stories that people with fouling problems were having Yamaha replace their carb...sometimes it made a difference? It seems like the pump is set DIFFERENT on all carbs...could this be the major starting problem and not a CDI problem. It would seem to me that it would. Just me thinking out loud.

Can't wait to race my bike this weekend with the BK mod!


My 01 started out the same way and since I did the BK mod and took his advice and started running a mix of VP 108 and 92 octane pump gas (1:1 ratio)I have been able to run the same plug for the last 4 races! Yamaha owes BK a bonus in my opinion!


Does it only foul at the beginning of the day (starting a cold motor), or do they go bad while you're riding?

My 01' 426 has done this since the first day - I've taken it back to the dealer twice for a check over but the cure was a new spark plug. Right now the jetting is stock except for the needle - I raised the clip once and now once again - so 2 positions. It ran great for one ride after that with a new plug - but for my next ride it was all fat on the bottom and at idle.

I know of four other guys in my area with 00' 426's with stock jetting and their bikes run great.

I don't wish to run a mix of race gas in my bike (I don't race, but I hammer out at the local mx tracks) Does anybody have any ideas? Thanks, again.

Sorry, to answer your question, it fouls at start up, and it fouls during a ride - I've pushed the bike home 3 times. It's definately fouling at idle to 1/4 throttle ... it's good if I can keep the engine singing but the track I ride most is pretty tight.

I have a friend who has a 2001 YZ426, this generally requires a plug change before it will start. Once hot the bike is 100%. Any plug taken out is black

Any suggestions?


If it's fouling while you're riding then there's definitely a problem. I wish you luck in getting it fixed. I would try pressing your dealer to get you a new CDI to see if that works (or swap your CDI with a friend for a day and see if it fixes yours and breaks his).


My '01 started doing exactly what you describe, but I noticed a pattern. If I don't start the bike without riding it (i.e. start it on the stand to show a friend then turn it off) then I don't seem to have this problem. I also recently changed brands of gasoline and I'll see if this also has an effect. My only other advice is a few shots of carb cleaner on the plug seems to fix it right up. I just rode all day yesterday on a plug that I fixed this way and it ran flawlessly all day (completely stock jetting, somewhere between 80-90 degrees, typical florida humidity). Hope this helps!

First thing I'd try is something simple, like adjusting the fuel screw. Each bike is different. Don't expect a Yamaha dealer to be able to adjust something like that correctly. You need to do the trial and error thing with the fuel screw. Screw it in 1/8 turn and try it. If it keeps fouling, try another 1/8 turn.

One other possibility is if the float level is too high. Make sure the basics are okay.


MX Tuner

I have experienced everything you have with your 01 426. I was fortunate to have a friends 00 cdi unit to experiment with. That was the first time I was able to ride without fouling a plug.I informed my dealer about this problem......they insisted the problem was not the cdi unit. I brought the bike to them to evaluate, they tried everything from testing the TPS, ignition, and finally the cdi.My cdi was swapped out with a cdi from another 01 426 from the floor...they claimed the problem was still there..I finally convinced them to order me a 00 cdi(under warranty) which I installed this weekend. No problem!No fouling!Thank you Santa Fe Motor Sports(it took them a month but they came through)

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