fouling plugs

I have a three week old 01 426 that ran great until I power washed it and cleaned the air filter. The bike has now fouled two plugs and has been popping and sputtering.I have very little oil on the filter and put fresh gas in it. Does anyone have any idea whats going on ?

Does the seat keep falling off also? (Inside joke)

I have the same problem. After a ride, I power wash it, clean the air filter, change oil, etc. Right after I clean/oil the air filter, my bike will start right up. If I try to start it up the next day, it won't start. So take out the old plug, kick the 426 over to clean it out, change the plug and it will start right up.

I believe that after you've cleaned the air filter, the excess oil runs into the air boot, then the great suction power of the 426 sucks all that oil into the carb.

This may be the problem of my starting problems, or it may be my jetting. Before winter came upon Michigan, I dropped the main jet to a 165 (I had 168 because that's what the manual said for a White Bros. exhaust I installed). I only started my bike once after I chaged the jetting, so that might have been my problem.

All in all, I think the excess oil on the filter is sucked into the carb. I'm going to start wrapping my filter in paper towel overnight after I clean and oil it. That should give it a chance to to be sucked up and off the filter. Then I'll just put the filter on the next day.


YZ 426's suck!(all the air they can get)

THROTTLE JOCKEY......because all golf courses should be motocross tracks!

[This message has been edited by teamtoxic (edited 02-06-2001).]

Ha! Ha! Ha ! It looks great when it comes off in mid-air.

Originally posted by YZThumpa:

Does the seat keep falling off also? (Inside joke)

[This message has been edited by thumpdaddy (edited 02-06-2001).]


Thanks for your reply. I just can't believe that a four stroke is going to be a problem like this. I bought the animal thinking that it would be essentially maintenance free. Fouling plugs is the last thing I thought I would have a problem with. I hope I don't regret buying it because I was really tempted to buy a two stroke. I think all this jetting stuff is crazy. This bike should be ready to ride.I owned two strokes years ago and I don't recall having to tinker with the jetting as much as people are with the 426. I love the bike. I hope I get it all sorted out. I think i'm going to contact the dealer about this plug fouling thing.If they have some tricks to this I will be certain to post it. Thanks again.

[This message has been edited by thumpdaddy (edited 02-06-2001).]

If you are fouling plugs in a YZF it is likely not jetting related. It is much harder to foul a plug due to rich jetting on a YZF than any two stroke (accelerator pump abuse aside). So if you are fouling plugs I’d suspect operator error first, ignition second. IMO, judging only from my current jetting on my ’00, the ’01 is jetted fairly well but a little lean, not rich. It seems just about everybody, even in places like Denver and Albuquerque (both 5,000 ft.) is going richer on pilot, main and sometimes needle.

Read the Fouled plugs = foul mouth post, three or four other guys had this problem and it looks to me to have been caused by the CDI.


I agree, I think it is an ignition problem. I called a dealer today and spoke with the head mechanic. He is going to try another CDI just as soon as I can get it to him. Also, he did mention something about adjusting the carb float. I told him to go right ahead but to try another CDI first.

Not trying to be an a$$ here, but you do know to NOT turn the throttle EVER unless the motor is running. The carb has an accelerator pump like a car and will pump fuel into the motor.



86TT225, 99WR, WR timing, throttle stop trimmed, air box lid removed, White Bros head pipe, silencer and air filter. Odometer and headlight removed. Moose hand and mud guards. YZ stock tank and IMS seat. Renthal Jimmy Button "highs" and Renthal Soft half waffle grips. AMA, SETRA.

Agood way of stoping the fouling is to buy and extra filter. Load it up with oil as you normally do but let it sit in a plastic bag for a couple of days or until your ready to change the old one. Also after you power wash it, take an air hose and blow some air through the small hole thats on the side of your engine just below the spark plug.

Dont foreget to remiove the spark plug cap first. This will help get rid of any water that may be in the well. Ive hade my 426 for over a year and have fouled two plugs. Both were my fault as I twisted the throtle after the bike had sat awhile and put to much gas in the bike soaking the plug.

Glad to help. Have fun, can't wait to rip some dirt, as soon as the snow is gone, crap its snowing again.


Thanks for the advice. I think I figured out what happened and it wasn't the CDI. The day after I posted the trouble I was having, I changed the plug again. Since that plug,it has started on the first kick everytime.

A few things that I think could have caused the troubles were:

1. Cleaned the air filter and did not let it dry.

2. Put too much oil on the filter.

3. Had some water in my gas.

4. Did not blow the water out the spark plug area.

I think had I avoided the above mentioned ,I wouldn't have fouled plugs.

Just a note on an old topic.

After washing the bike after the last ride of our season(which was a sloppy day)I fould a plug on my 3 ride old bike starting it in a heated garage. I never thought of jetting cause the thing ran so well when it ran. Also i hadent changed the filter yet. What i did do was unplug electrical conectors and was amazed how much mud and water got in there, especially the TPS conector. Be sure to squeeze some electrical silicone in those conectors.

A little TLC and a new plug and she growls again.


Originally posted by thumpdaddy:

I have a three week old 01 426 that ran great until I power washed it and cleaned the air filter. The bike has now fouled two plugs and has been popping and sputtering.I have very little oil on the filter and put fresh gas in it. Does anyone have any idea whats going on ?

Hey Thumpdaddy!

TeamToxic has it exactly right. Never start you 426 with a freshly oiled filter. The air hungy beast will suck that oil right into the carb and foul your plug every time. I went throught he same thing when I bought my '00. I took this advise and I have'nt fouled a plug since.

Just talk to dealer on my 01' 426 and the throttle position sensor is bad. Yamaha is going to replace it and thinks this will solve my problem. I guess the tps senses the throttle position (duh!) but also the gear your in and relays this info to the CDI which regulates flow from the carb and can cause fouling. Anyway it looks as if Yamaha has seen the light and realizes this is a design problem and is fixing mine. Good Luck!

The TPS only affects the ignition advance or retard according to the map. It does absolutely nothing to the "flow" of the carb...air nor fuel.

Now, that I have fouled my first plug I can shed some light here.

I have been running the stock NGK CR8E plug that came in my bike for three weeks now. My bike has about 15 hours on it.

I went to ride it yesterday morning and I fouled the stock plug. I am pretty sure I turned the throttle not thinking. So I pulled it out and replaced it with the extra I got. NGK CR9E.

After replacing it started first kick. After riding a couple hours, stoping and starting in very wet conditions I let my riding buddy try my bike. He went very slow on it. After he was done I could not get it started.

When I got home I changed the air filter. I have three that I keep ready. Swapped the gas, and check the carb for dirt inside and out.

I then removed and cleaned the plug, CR9E. No go. Then removed the plug again and heated on my stove. No go. So I trashed that one.

So, I took my stock plug cleaned and heated it real well on the stove and presto fired first kick. It also, started second kick this morning with choke.

What did I learn?

1. Do not turn throttle unless it is on

2. CR9E it too cold for slow speeds

3. You can foul plugs lugging it around.

I also adjusted my air screw from 1.5 turns to 1.75 turns. This really cooled off the exhaust pipe.

I agree on the freshly oiled filter issue. Keep in mind too that you really don't need a lot of oil to make the filter work.

And, I've also noticed that I get some water down inside the spark plug hole after long washes. Not to say this is your problem, but just check it out.

It's also a very good idea to always blow off the bike with compressed air and then start it after each wash.

And I've never had a fouled plug on this bike - so don't this topic scare too many people off.


I've never put on a freshly oiled filter and never had a problem. I keep spares that are cycled (ha ha) into the bike which have been previoulsy oiled (at least over nite). Something I read in MX Action. Additionally, my bikes are only washed with a cheap (low pressure) pressure washer, taking care not to excessively blast the thing.

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