Fouled plugs = foul mouth!

Forloop: Twin Peaks Powersports took care of everything for me. The bike was new and I wanted Yamaha to be responsible. Call TPP at 800-864-4269 and talk to Gil for details. I know Gil told me the CDIs were the same P/N from 2000 to 2001. Hope you get yours sorted out, mine runs great now.

I did not see the P/N myself, only have what Gil said to go by. I still recommend calling TPP if you have questions. They did bulldog the problem down for me, an I did not even buy the bike from them.

I called my yamaha dealer and the part # for 00 and 01 cdi was not the same and I also read that they added some changes in the cdi to make the bike start better in 01. I think the only way to confirm this part # issue is to see the part # from the dealer for yourself or call yamaha.

I have 00 and never fouled any plugs, but couldn't take the little hestation off the bottom. So what works great for me is 38 pilot and the clip on the 5th position down,stock main and the air screw I do a ide drop with stock exquast.No more bogs and the plugs perfect.And I run a bigger main in the sand and gearing is diff. that my .02


I got the CDI in on Friday and replaced it in my 2000 WR. After a brief ride the plug came out black. I checked and found the TPS coil resistance at idle was off. It read 1000 ohms and target range is 650-750 ohms. So out came the dremel and I took those tamper resistant torx head screws out to adjust to 700 ohms. After the adjustment I have noticed the bike starts easier and seems to be more responsive to the hot start button but the plug still comes out black despite lean jetting. The only check I have yet to make in the electrical department is the neutral switch. If this tests out OK...the whole ignition has been gone over and tested for correct readings.

what does it matter if your plug is black? Im sure my bike is running a tad lean and the plug turns black in five min of hard mx sx riding but the bike runs ok

Yamaha finally replaced the cdi in my 01 426 and it's run perfectly since: no fouled plugs, no periods of poor running that disappear after a few minutes. I'm running a accel pump diaphram from an '00 426 that made only a very slight difference when wicking it open (but slightly crisper so I've left it in). Jetting is now: 45 pilot (3/4 turn out - it's getting warmer), 4/7 needle, 165 main. I tried a range of clip positions (5/7 was way to rich but 3/7 and 2/7 were maybe a bit lean) but returned to stock for best all around use. Haven't fouled a plug in months but we'll see what warmer weather brings. I agree with other posters that the cdi-as-culprit doesn't make any sense unless there is an odd temperature dependency that isn't seen at the factory. I had the cdi on the '00 fail and it seemed to run a little richer in the 30 seconds it took to go from good to complete failure but when it quit that was it.

what's the chance that the jet wash through the little hole is knocking out the plugs, as has been suggested above.

if you go over to the WR side you'll find "at last snap & 3/4 throttle response. in it i explain how i thought that the riders using 48 PJ's ought to be going up on the PAJ as well. this i did & the bike runs beautifully at 1/4 throttle.

i think that the 48 PJ has been responsible for SOME of the fouled plugs.


blbainb: Good to hear the CDI cured yours too. What elevation and temperature are you running that jetting at? Is your bike stock?

I do not know at this time if my plug fouling is behind me or not. The plug readings that I have taken have been in cold weather (30's F) and for short rides (2-3 miles). The true test will be when temps warm up here and after some longer rides. I have no problem tolerating a black plug unless as a result it goes into the popping, rough running/fouling mode. I am not assuming that the CDI was not the problem...the new CDI is staying in until more time can be spent riding and hopefully gaining confidence that it will not foul. I want a bike where I do not have to put in a new plug every 50 miles and have to worry about fouling constantly.

[This message has been edited by Moto-Mike (edited 03-13-2001).]

Just thought I would add a few more cents worth. I have done three things to my 98WR which have completely solved my plug fouling problems. The most important was to never blip the throttle until the bike is very warm. Start it and as soon as it will run without the choke (set the idle a little high) I let it idle for a few minutes and then just ease on the power until she is warm. Seemed like if I blipped the throttle (which is always tempting) the plug was hurt pretty bad and you could only sometimes save it by finding a hill to climb. If you have ever twisted the slide with your carb off you know why - gas shoots clear across the garage. Second, switched to a CR7E plug. I mostly ride trails and have never heard a knock yet so why not help the plug deal with that straight gas that is shot at it. Last - when it is cold out (<50 degrees or so) I cover one radiator over with a piece of cardboard. Since there is no thermostat I believe our bikes are waay overcooled when it is cold out.

Just my .02.



98 WR400F, YZ'd cam, YZ muffler

My '01 426 is currently jetted at 45 pilot, 4/7 on stock needle, and 165 main. This is at 5000'+ altitude with temps in the 50Fs. I'll be going to a 162 main and the 42 pilot as temps climb into the 60s and 70s.

It's interesting to read all the posts/opinions on jetting. However, I remember having cars with carburetors (no efi or throttle body, no hot wire/air sensors, no computers, etc.) and can't remember having to rejet or be careful about not using the throttle before it warmed up or any of the things that people are bringing up. A properly functioning 4-stroke simply is not very sensitive to jetting/temperature. It will not load up if you happen to use the throttle before it is fully warmed up. It should idle just fine warm or cold. Just like a car. You can improve performance by rejetting when there is a big change in temperature/altitude or exhaust system but in general it shouldn't be a problem. If your plug is consistently black/carbon fouled and you're using stock jetting then there's probably something wrong other than jetting.

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