Am I the only one to ever have problems fouling plugs????????????????????

I read this site alot, and I must be the only one to have ever had plug fouling problems....

Top end is tight, bike is mint, It has to be something that someone knows how to fix.......

The bike does'nt smoke, use oil or run bad.. until plug fouls.....

PLEASE HELP MEEEEEEE...................


What is that sh??.........

I ride hard.. last I knew that is how you ride a dirt bike......

Just a suggestion...are you putting too much oil on the air filter?

[This message has been edited by Kermit (edited 10-18-2000).]

I used to foul plugs on a regular basis untill I switched to splitfire one heat range cooler (SF430B). Have been using the same plug for over a year, and I have over 15 races on it this year.

I changed it about 3 weeks ago (general maintenance) and it was perfect (tanish brown).

Keep in mind, I have a bone stock 98YZ.

Hope this helps,



'98 YZ400F

'01 YZ250F

'98 CBR600F3

1. What fuel are you using? High Octane or regular?

2. What type of plug are you using? Single or double core?

3. When you oil your filter, do you oil just before the ride or the night before? Do you let the filter sit outside the box or in the box?

I think you will find it may be one of these three problems. If your mechanic is telling you it is how you ride, ask him to explain himself. If he cant, go else where.

Good luck


1 high octane

2 ngk cr8e

3 I leave my filter sit out all night to dry..

when I say I foul a plug I mean in like 4 miles...

That is with a 168 main 45 pilot dtm 3# clip from top.... only baffle out and air lid off...

Now when I put Needle on 2# clip with a 165 main can run long time but plug reads lean. then I'll put 168 main in runs for 20 miles then plug starts to act up. If I turn my gas off and run till float bowl is emty then turn gas back on runs fine for awhile then I have to do the same..

I can believe. I fouled plugs constantly when I got my WR.

But I was running at high altitudes: 7000 to 13000 feet in Colorado.

Dropping the clip to DTM #2 and dropping the pilot to a 42 on real high rides totally eliminated the problem.


I can relate to your plug fouling...I fought this problem for some time. I always make sure I have a fresh (brand new) plug (CR8E)in before a race. One thing I've read and experienced is that these bikes run very well even when they are running very rich. What I've done is progressively leaned out the jetting until the plug reading went from black to tan. I have not experienced any problems since getting the jetting dialed in (I still replace the plug prior to a race as a precaution).


2000 Yamaha WR400F

1988 Honda NT650 Hawk

1977 Suzuki RM250B

1974 Honda XR75K1

Sorry about the above reply. After reading through your reply I understand your fouling is'nt due to rich jetting. Next time I'll read the follow-ups. :)

I have the exact same problem, bike runs great until it fouls, then comes out black and somewhat oily. I have removed airbox lid, aftermkt exhaust and 1 size richer on pilot and main per thumper racing recommendations.... Anyway, if you guys read this, I am 2 turns out on air screw, would going back in maybe 1/8 turn at a time be a good idea? Then test from each increment. :) Taking a plug reading is a PAIN, but I love my 99WR anyway.

Yeah, I have a simular problem. The bike runs perfectly between 50 km to 400 km on a new plug, then it starts to foul plugs. When the motor is cold it backfires, but if I just run it like that until engine is warm, the backfireing stops and it works perfectly. But then I have the same problem when engine is cold again (next ride). Always starts on first or second kick (both cold and warm). Plug reads a little rich, but not much. IMHO you are all experts, and I would appreciate suggestions on how to solve this problem. I have never been involved in jetting before, but I suspect that it´s a jetting problem.

It´s an WR400-99 euro-model, completly stock.


´99 WR400F

´98 CBR600

Check your accellerator pump rod at the pivot. The rod should be centered in the hole. If it's not adjust the metal tab closer together or spread it with a screw driver until the pump rod is centered. You could be suffering from premature fuel injection....into the carb of course!

I'm assuming you never turn the throttle with the motor shut off?



86TT225, 98CR80, 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.

Hi Bill

OK, I will look into 'accellerator pump rod'. Only problem is that I have my bike about 650 km from where I live now (and I have a race coming up in two weeks), will bring it to me next weekend though.

Any other ideas are appreciated.

You assumed right about the throttle, I never touches it with motor shut off.


´99 WR400F

´98 CBR600

Can't Believe,


There are many possibilties for plug fouling. Here are a few things to check on your carb and jetting.

1)Pilot screw and jet- This is a fuel screw, turning it out is richer. Turn it in 1/4 turn at a time checking for responsiveness. If you go to a bigger pilot jet expect to have to turn it further in also.

-Can't Believe, how many turns is it at?? Make it less than 2 turns with a #45 if you're fouling. -Mark M, If you're fouling on a #48 at 2 turns, set it back to 1 to 1 1/2 turns. Linmag, whats yours set at? Don't use a #50 pilot jet with any needle ending with M.

2) Float level - check that it is not set too high. Make sure the floats are good. Look for defects on the needle and seat. Clark suggested running it 1mm lower than stock, good idea.

3)Accelerator pump setting (Bill's idea) - Make sure it has the proper delay, .6mm on the '99 fork or about 1 screw turn on the '00. Low speed operation will have it squirting extra fuel too often if the delay is not there.

4)Needle selection and clip (Bryan's idea) - DTM or DXM - These needles have a small (rich) straight diameter. The straight diameter affects fuel from 0 to 1/4 throttle. Low speed operation can get too rich, like on downhills or tight trails. A leaner clip position will help but is mostly for a higher throttle setting. A typical setting is DTM#3 or DXM#5.

DVP has a larger (leaner) needle straight diameter and will reduce the richness below 1/4 throttle. Try DVP#4.

DVR is leaner yet and is the standard YZ400 needle. Next step would be DVR#4.


100) main jet - Don't even think about it! The fouling is not from your main jet. With the throttle restrictor you could take it out. Just to make the point I rode my bike 20 miles with no main jet. It ran fine everywhere except full throttle.

James Dean :)

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