Additional Fouling

What's up fellow thumpers.

I have a question concerning a fouling problem that I've been dealing with since I purchased my bike almost 3 months ago. After many many jetting changes, both by myself, and then letting my local Yamaha shop "CO Powersports" giving it a whirle,I still am loading up and fouling plugs. :)

I'm not fouling plugs nearly as bad as it was at first. The bike runs good, sometimes great. But non the less, I usually cannot go more than 2 rides on a plug. My backgound is mostly with a 2 stroke, and to me a black plug denoted a rich mixture. On my '01 426, I'm getting the very common sooty black plug, "Carbon Fouling" I guess is the proper term. Again, to me this seems that the bike it to rich. My jetting now is much leaner than how it came stock.

Here is the twist. Last weekend I was riding above 14,000 feet here in Colorado. Except for just a very minor loss of power, which is to be expected, it ran awsome. As I dropped down in elevation I started to get a lot of popping, and if I needed to whack the throttle just a little, it would load the plug up. I have been told that the popping is a sign of to lean of a mixture. THE MILLION DOLLAR QUESTION.... WHY IS MY PLUG STILL BLACK AND CARBON COVERED IF MY MIXUTURE IS TO LEAN. Any help would be appriciated. Today, I am expecting a call back from the president of the shop here in Boulder, hopefully with some kind of answer as to how my problem is going to be fixed.

Thanks a lot, Later!

see if your dealer can test your cdi. If not maybe they have one that you can try out. The cdi has been a big problem with most plug fouling.

People have had CDI problems with these bikes from what I read on here. I agree with bjscheel, see if the dealer will swap you a CDI at least long enough to see if this is the problem.

Also whatever speed the bike runs before you shut it off to look at the plug, that is the condition that you're testing. For example, if it was idleing the you're seeing the result of idle mixture. If you want to check the wide open mix, hold it open in some gear and then hit the kill switch before you let off the gas. etc.. Hope this helps..

Co powersports is clueless. They were unable to resolve my plug fouling problem, or my friend Jeff Brattons problem either. After Twin Peaks fixed mine with a CDI swap, I went in and told them that was the problem. They still don't believe it. They will just jet your bike and tweak the fuel screw until you tire of bringing the bike in and go somewhere else.

I talked to you about this, didn't I? Several people have called me about this problem and I can't remember if you are one of them.

The popping on the YZ 400-426 is NOT a sign of lean jetting. That is a misnomer and bad information. I raced my YZ 400 in an organization in Arizona called Whiplash Motorsports and made many friends with YZ 400's. Most of us ran White Brothers E-Series pipes and they all popped. I moved to Colorado, rejetted for the altitude, and mine still pops on deceleration. I have never fouled a plug and it isn't running too lean, either. Forget that information. I run a 160 main and the rest factory on my 99 YZ400 and I ride at 9000 + feet.

I'm sorry I can't help more with the solution to your problem, but hopefully this eliminates one aspect.


Colorado is correct about the popping, I have 00 426 also with the WhiteBro E-Xhaust.

Guess this, it usually pops after the thrid or fourth ride when the air cleaner is loaded with dust/dirt. I would replace the air filter with the spare. The poping would disappear till about the 3rd and 4th ride. So now everytime it pops I would swap air filter and also do an oil change.

My fouling problem was totally solved after talking to Brian Kinney, Tim Ferries engine mechanic.

My prob was carbon build up in the exhaust port and on the top of the piston which collects moisture and can even short the plug out.

This build up was on a 6 month old 01 426.

After I started running race fuel my problems are long gone and the back of my silencer on the inside is totally white, none of that nasty black build up.

Just remember this bike is high performance and they would have sold alot less of them if they said you had to run quality fuel.

My 2 cents.


Jaybird - I'm taking issue with the race gas aspect.

This bike was explicity promoted as "designed, developed, and marketed to run on pump gas". That is the verbage used by Yamaha in the press releases, and the magazines. IT also is part of the reason I pointed to the YZF over the YZ when telling the dealer which bike to prep.

I do not disagree with the probability that race gas can run cleaner, etc, etc, but that simply is a moot point to most of us riders. Premimum gas is expensive enough for most of us, regardless of the fact that race gas is way more expensive and it IS NOT easy to locate for everyone.

Another marketing plunder by Yamajunk??

Just my $0.02


I've heard of people fouling plugs by leaving the choke on during warm-up. As soon as you get it started, shut the choke off. Not sure if this is contributing to your problem, but it is worth noting.

Dodger, for the 6 months I have owened my bike, it has suffered from the exact malady you describe. I have tried every jetting combination possible. The jetting changes I made affected the power band of the bike as I expected, but never cured my problem. It has now gotten to the point where the bike will barely run ( starts easy, then runs crappy for a couple of laps, then runs a little better, then fouls a plug, etc). I am waiting for and hoping the CDI fix will work for me. If not, I guess I will move on. This has been the most fun bike to ride and the most frustrating to own I have ever encounterd!


My bike would wet foul on start up not foul after it was running, so this is diffrent from people who are fouling after the bike is running.

Peoples bike that fouls after they are running is either jetting or CDI problems I would suspect.

As far as the availability of race gas you can get it any where here in the great USA.



Originally posted by jaybird67k:


My bike would wet foul on start up not foul after it was running, so this is diffrent from people who are fouling after the bike is running.

Peoples bike that fouls after they are running is either jetting or CDI problems I would suspect.

As far as the availability of race gas you can get it any where here in the great USA.



My bike had the same problem. I now use a 50/50 race gas mixture. I also had to rejet mine. 3rd and 158 main.



01 YZ426F #85 Vet C

I made a long reply to this problem in another post but here are another couple of 'pearls' (turds?) of wisdom.

Popping does indicate lean jetting but only leannes of the pilot circuit - not the needle and main. Yamaha typically sets the pilot circuit up on the lean side and it runs better with another 1/4 to 1/2 turn on the pilot screw.

The bike runs fine on premium unleaded. Rumor is that some race gas will damage seals in the carb - but who really knows. Excessive carbon buildup can be due to jetting and riding behavior.

Make sure you do a full throttle burn and engine kill before checking the plug. My bike was and is too rich off idle (burbley, rough) but a proper plug check shows it is jetted properly at mid and full throttle. The region surrounding the insulator is always black/sooty but the insulator is white/tan - unless I just let the bike idle and then check the plug.

A new cdi cured some problems but not the jetting problem and a new carb didn't change anything. Your problems could be different, however.

I think the 'fouled plug problem' is really a result of Yamaha's attempt to cure the 'bog' problem of the '00 426 when rapidly opening the throttle. Maybe due to a combination of cdi and pilot circuit mods they over did it - the bog is gone but a new one emerged. If so, the '02 are probably a happy medium.

It's your CDI

double ought 4-2-6

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