Fouling plugs!

I fouled three plugs in one day, two of them brand new! This was in about 5 hours!

The bike has been running bad on short rides around home since the weather broke here, but not dying on me.

I have a 98 WR, with YZ cam timing, wb e-series pipe with 12 discs, 178 main, 48 pilot, air/fuel screw 2-1/4 turns out, stock wr needle(don't remember position), airbox snorkle removed(lid still on), clean airfilter.

I am in Michigan, alt below 1000ft(I think), temp about 60f.

Bike seems to run good except pops on deceleration a little(lean?). After a while starts missing thru-out rev-range. Seems to make no difference whether bike is hot or cold. Problem seems worse as the outside temp dropped in early evening(lean?).

I tried screaming it, & lugging it. The plug always comes out rather black(rich?).

I am thinking of switching to a 50 pilot to solve the popping on deceleration & going to one of the leaner needles. Maybe going down to 175 main jet.

My question is, I have had two-strokes that were run too rich, the plug was very black & wet. Clean the plug & they would usually run again. Mine won't!. Even tried one of my old plugs in another WR after I cleaned it. The other bike would barely run with my old plug.

Does the popping on deceleration damage the plug in some way? What do you think?


Roy unless I'm wrong I think that you stock WR needle might be the problem. I run my bike the exact same way you do but I'm running a stock YZ needle (DVR needle), 180 main and have my screw set at 2 turns. I live in Southwest Ohio at about 500-700 feet. I haven't fouled a plug yet. The WR needle is much richer than the YZ needle. I did have a little hesitation off idle, but I ordered a DTM needle which is one step richer than the stock YZ needle. Clark Mason made some suggestions and he has not been wrong yet :D . I think he really works for KEIHIN and designed the FCR carb but he won't admit it. Make sure allof your vent tubes are clean also. Good luck. :)


00'WR400 75'Yamaha 250 enduro

White e-Series S-bend 12 discs, Yz timed, rejetted to Clark specs, throttle stop cut, lid removed, Topar Racing top clamp, Tag Metals T2 bars, Scotts damper, Devol disc guard and frame guards, lights removed, Cr routed Fastline brakeline, Acerbis Rally pro hand guards fastened to the top clamp, Gel grips Live in Southwest Ohio

Hi Roy.

I love fouling plugs too. I then like to take the bike to the dealer to complain only to have them tell me I'm not running the engine correctly and not starting it right.

Sorry, just venting.

If you are running the stock WR needle (DTM) at the stock clip position (#3) at your altitude, I would think the bike would run OK. Normally when folks go to the YZ timing they use the stock YZ needle (DVR) or go to the needle suggested by Clark, a DVP. However, your setup is roughly the same as running the stock YZ needle (DVR) in the 4th clip position and the DTM gives you a little more richness on the bottom to make your 48 pilot act like a 50 with the DVR needle which should be OK at your altitude (did you follow this?)

You could try and either use the DVR needle for your YZ timing or reduce your DTM needle to the 2nd position. But, KerryT here in Denver successfully runs his YZ timed WR all the way up to about 900 feet with the WR needle at clip #2.

From your symptoms, it sounds like your pilot is OK or a little lean if it's popping. And you didn't say you were fouling the plug at full throttle which is the only place the main jet matters. So, if your problem is with the jetting, it's in the needle area.

I'm wondering if you have trouble somewhere else though. You might want to go through the procedures of checking your carb float bowl (I haven't done that yet). I believe others have had problems that have the symptoms of bad jetting when it's really been a float bowl problem.

I would try adjusting your float bowl. If that doesn't work, reduce your needle clip by 1 position or (I'd do this) go to a DVR (stock YZ) needle at 4th clip position. At your altitude, this setup will probably be a little too lean but if you can keep from fouling, you can start richening later. I'd do this one step at a time before messing with your pilot or main. I successfully run a 178 main at 4000 to 5000 feet with YZ timing and haven't had any problems. I would agree that your 48 pilot might be a little lean for your altitude but I'd wait on that adjustment until you get your fouling problem worked out.

You aren't goosing the gas before starting the bike are you? Sorry, had to ask just to be sure. (My dealer didn't believe me when I told them I never do that).

Let us know what you come up with.



Obviously your are runing rich on the bottom. If you insist on using the DTM needle then go back to a 45 PJ and 1.0 to 1.5 turns. With YZ timing it would be better if you changed the needle to a stock YZ DVR needle and ran it in the #5 position from the top and used the 48PJ starting with 1.5 turns and only work with the turns on the fuel screw. The DVP needle works well in the #3 or #4 postion. With a 45PJ, 48PJ and a DVP and DVR needle you should be able to dial it perfectly.

MY recommendations would be as follows


Fuel Screw 1.0 to 1.25

DVP #4

178 to 182MJ

The MJ is not causing the fouling problem unless you are running WOT all the time and I seriously doubt it. Remember a little poping on decelleration is ok as long as the throttle response is ok everywhere else



so you think he's running too rich on the pilot caused by the DTM needle?

I figured since he is popping on deceleration that he was OK on the pilot but too rich in the middle (needle clip).

But, according to James Dean, the DTM in the 3rd position is the same above 1/8 throttle as the DVR in the 4th clip position which should work fine at his altitude.

It seems like his jetting with the DTM should be adequate at his altitude based on other bikes although, I agree, optimally he should go to a DVR or even better, DVP.

I'm not arguing but rather still just trying to get an understanding of all this.


[This message has been edited by Bryan (edited 05-01-2000).]


You have gotten the best jetting advice available on line. Clark is the resident expert here and plug fouling due to jetting most often happens on the low end.

My question is how did you store your gas and could it be stale gas?? You said "on short rides since the weather broke". Was the bike stored in a warm place? Was it cold in Michigan last winter? (silly question) How did the bike run last summer? any mods since then? Even if you used fresh gas, clean out the carb and check the floats. Since this is a '98, it certainly hasn't always run this way, but the nozzle(needle jet) that the needle slides in can eventually wear and richen the low end. 2 years of riding is enough to make a difference. The size difference between a DTM #4 clip and a DVP #4-clip is less than .001 inches, but is significant for low end jetting. What was that clip position anyway? What type of riding - tight woods or more open?

Also, Clark and Bryan, I went up to Kamloops BC last weekend and rode 175 miles with a tour guide named Guy Perret of "Canadian Offroad Holidays". It was a great time, some fast, some very technical, and lots of challenging hills. He wore us out. Guy and his assistant were riding Husabergs(501 and 400). It's a great playground for thumpers right from his shop and bed & breakfast. The Canadian exchange rate makes it a bargain. You can see Guy and the area in some of the Terrafirma videos. Check it out.

Stock WR400 clip position is DTM #3.



The DVP is a leaner needle it has a larger diameter straight section and will be slighly leaner. The DVR will be even leaner. Yes you can come close with a DTM for wider opening throttle positions BUT the DTM will always be richer on the bottom and this is typically where you foul plugs with this bike.

I'v had plugs start to foul at 3500ft, 48PJ 2.0 turns, DVP #4 and a 182MJ. Went to 1.0 turns and problem completely went away. This only happed in slow speed down hill conditions.


Thanks guys! I will try your settings before I ride again & see what happens.

A little history, last year I switched to YZ timing & went to a 175 main jet. The bike ran good all year until fall & cooler weather set in. Then the bike seemed lean so I installed the 178 main & 48 pilot(up from 45), the bike ran ok for the little riding I did.

This year my half dozen short outings were in cooler weather(40-50f). This last Saturday I made a trip up North for the day & the weather was warmer(60f). The bike ran fine down the roads we traveled, but acted-up in the trails(single track, 2nd gear stuff). We road about 60 miles total.


Thanks Clark.

I guess I ruled out the possibility that he is fouling his plug on the pilot since he said he's popping on deceleration which indicates a lean condition and didn't mention that it was only happening on down hills at low or no throttle.

When my pilot was too rich this is when I was fouling plugs constantly. Long downhills with occasional throttle blips. Leaner pilot fixed it finally.

At any rate, it sounds like going to a DVP needle should solve the problem. I ordered mine through my local dealer and it took about 1 week. Or you can probably order one through Baja Designs and it might arrive quicker. Your local dealer will probably have the DVR needle in stock though if you want a quicker fix.

Good luck Roy,


Update: Last night I cleaned my plug, it was pretty black. I Then changed the needle from 4th position(from the top) to the 2nd. I also turned to fuel/air screw in to 2 turns.

The bike ran fine & the plug looked good after 10 minutes. Now I just need to order a new needle.

Thanks for all the input guys!


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now