'98 WR400 fouling plugs after new parts

First things first, I was not the original owner of the bike, but I've replaced just about everything but the carb. My last major issue was a broken cam chain, and the replacement parts included: Manual cam chain tensioner, new cam chain guides, new cam chain, and I re-shimmed the valves while I was at it among some other things I'm sure I'm forgetting.

The bike is a 1998 model and has a pipe, grey wire mod (YZ timing?) and I'm assuming a re-jet by the previous owner, as well as a zip-ty(?) fuel adjustment screw (THIS IS THE WRONG ONE as it was designed for newer model and is wider so the large hex head at the bottom of the carb had to be clearanced to allow the screw to fit.)

When I pulled the plug to start the work it was golden marshmellow brown and I was pretty much tickled pink to see that. After assembly, the bike ran great minus some noise which I determined was loose tension on the tensioner so I set it to mimic the OEM auto tensioner. After that I ran it and I was generally pretty happy.

After a little tweaking here and there with a few short rides I took it on a ride with my roommate adjusting things a little bit including fuel. We were out for a decent while and things were going well. On the way back doing about 50-55 on the road the bike cut and violently puttered out and quickly died as I pulled over. I could kick it back over and it would start but again would quickly die. I let it set over night and tried again in the morning. It turned on and it seemed like it idled perfectly for a few minutes until it warmed up then would sputter out backfiring and die.

I pulled the plug and it was pitch black with carbon. I went and purchased a new plug, installed it, and leaned out my fuel screw. The next day I rode it to work and on the way back ~7miles or so it started to sputter/backfire violently so I pulled over, leaned the mixture out some more and limped home sputtering but not as bad.

A few busy weeks go by and I am home from school and I got a new plug in the bike and leaned it out until the screw was pretty much bottomed out on the clearanced hex head on the bottom of the carb. The bike ran like a dream but after ~5 miles or so it crapped out and left me to push it home. Luckily someone saw my trail of sweat and offered to give me a ride the rest of the way. I have yet to pull the plug but I bet it's carbon fouled.

I'm wondering what is going on? Could improper timing cause this? Did the new cam chain and valve clearancing alter the motor enough that the fuel scew can't thread in enough before bottoming out on the bottom of the carb (if you recall the newer models have a different shape to them). Is my jetting WAY off? I'm a little lost and frustrated.

Thank you for any tips you can give,


P.S. The plug is a NGK CR8E if I'm not mistaken.

Fuel screw I have "fits Keihin Pumper Carburetors ('FCR'). It fits all YZ/WR 426F, 250F and 450F bikes, as well as the YFZ quads. It does not fit the YZ and WR 400Fs"


The one I should have: medium_FMS02-web.jpg

I just remembered that I used some blue RTV silicone sealant to help the bottom cover of the carb where the float is seal a little bit better. Maybe some broke off and it's blocking up a passage in the carb? Or is there no way that a blockage could cause a rich condition?


That fuel screw you're showing only really affects the 0 to 1/4 throttle input. If you're fouling plug and ending up with a black carbon condition means you're running rich. Your problems could be as easy of a fix as changing the needle clip position, to swapping out the main and/or pilot jet. Somewhere on here there's some good reading on threads and posts to jetting a FCR carb using chop logic to tune the top end, and using that fuel screw to tune the bottom end. You really shouldn't have to put any silicone sealant on the float bowl, the oring is cheap and as long as you're not overtightening the four little screws you should never have an issue where the oring flattens out.

Hmm, I'm just wondering why it ran perfect before the new parts. And now it's way off. Quite possible I tightened those screws too tight, but I only tightened them down because it was leaking :thumbsup:

You've done all this work and are "assuming" it's been rejetted? Think you answered your own question, time to pull the carb and go through it. I'd be looking at needle seat, float and your float valve and slide. The older carbs on the 400 are known to be failure prone.

If your plug is truely carbon fouled and not fuel fouled, you have other issues including timing as a possibility or valves not seating correctly. More info on exactly what you have done would help as well. I'd start at the carb and check timing after that.


So long story short since I typed it all out and then lost it:

Purchased bike with: grey wire mod, yoshimura RS3, a sticking clutch, and bad valves.

I purchased/replaced: head work with new one piece stainless valves from MXtime, UNI air filter to replace what I believe was the OEM gray one, and a Barnett clutch basket and tusk clutch.

The bike was ridden for a while as my commuter and I had no complaints. After a road trip ~6hours highway round trip, I could tell something was wrong. I checked the valves and they were pretty far off spec and I also luckily noticed that some of the cam chain links were broken and had sliced up the guides.

After a long process of waiting on parts I installed new cam chain guides, a new cam chain, a manual tensioner, and re shimmed.

Then the problem started. It rode really well for a while 1+hour, now every time I put a plug in, it will foul within 15-20 minutes of riding. Would timing do this? Or would it run bad all the time if I was a tooth off?


Edited by nnamssorxela

Finally found some free time today and I pulled the carb off to take a look.

From what I could tell I have a 48 pilot jet, an 82 starter jet, 170 main/needle jet, and there is one on the outside of the carb on the side that is a 90...is this the pilot air jet?

The needle is the JD blue needle (I am zero elevation) on the 3 clip down as suggested by the instructions. Do I need the red as I am high heat and humidity? (my other residence that the bike sees is ~1000ft.

Also...:thumbsup:...the rtv silicon had squished out when the seal was made and it possibly could have been coming in contact with the float. Whould a sticking float cause the problems I mentioned?

My mods pertaining to jetting are an airbox with no lid, uni filter, full exhaust...grey wire mod?

Thanks again,


Tonight if some free time falls into my lap I'm going to re-assemble the carb and feed it some fuel. If it doesn't leak (doubtful) I will throw it on the bike and see if the silicone sticking the float was the problem.

If it still leaks I'll buy that $30 gasket :thumbsup: and maybe a lower pilot jet as it seems that most of the wr400 folks are staying with a stock 45 pilot according to the jetting database. The 170 main seems ok as well. Not sure what the other 2 jets are though, but they seem higher than what most were running on the database. (the starter is with the main and pilot, and the pilot air is outside on the side of the carb?)

Thanks in advance,


Sticking float could have been part of the issue.

Thanks for the tip. I came home and crashed last night so no spare time. TONIGHT I will do some work.

Just ordered the bowl gasket, starter rod cap, stock 45 pilot in case, and the proper fuel screw. Can't wait to test it out!

ok, so the new bowl gasket and fuel screw were installed. the screw was put all the way in then backed off 1.5 turns. Replaced the carbon fouled plug with a new one (ngk cr8e as per the manual) Started first kick. I tinkered with it at idle and that seemed to be about right. After blipping the throttle a while and messing with the screw some more it didn't seem to affect anything. Then it backfired with flames a few times coming off throttle messed with the fuel screw a little more going back and fourth then I decided to call it a night. Today I took it out to see what the deal was and the same choking out and backfiring occurred. Willing to bet another carbon fouled plug.

At the rate I'm using up plugs I could be parting this thing out and putting payments on a new bike. I'm getting pretty frustrated...what gives?


EDIT: Going to pull this thing apart soon if I get no replies and check timing....I bet it's at tooth off...I've NEVER been able to get all the cam marks to line up with the deck though and last time I put it together I was with someone "familiar" with the motor and they had me set it up at least a tooth off from what I normally do if I recall correctly. Is that a probable cause to this problem?

So here's what the timing looks like. I took a couple of different angles to give you guys the full view:

"I" on "HI" lined up with the marks


From straight on, close to lined up with the deck


From slightly above to see the outer marks


Fouled plugs...now I have one more to add to the pile


Open to any and all suggestions...


Edited by nnamssorxela
Fixed pics

Nothing? can the mods move this to the fuel and jetting section?

I had to remove the duplicate post you made in the jetting forum as it is against the rules to cross post.

You say the bike starts fine and once it heats up, it fails? Also, it used to run fine then suddenly, out of the blue, it did not. Based on that, it is not jetting. Jetting dies not suddenly change.

First, put the cam timing back and test. Also, while you are there, recheck the valve clearances.

If the problem persists even with the cam timing back, my money is on a failing stator, possibly from a bit of broken cam chain.

Sorry, bad call on my part about the double post.

Bike starts first kick, idles fine, and revs fine. After a little riding or revving after it warms up it backfires and sputters so much it is literally unrideable. With zero throttle it will sputter once or twice then die. I can post a vid if necessary but it will be at night. Will check clearances tonight and move the intake cam counter clockwise 1 tooth? I will post a picture of that as well. This is my first bike and it's still foreign to me despite having it for a few years, most of which it has sat in my room or garage waiting on parts or broken.

The new fuel screw is a pain to adjust but I'll set it at 2 turns out and see where that puts me. I might just richen it to where the header is just under red hot at a standing idle as that has worked in the past. :eek:

This is getting frustrating. I just want to ride.

I found this page: http://www.thumperfaq.com/yz_timing.htm

Which talks about yz timing. I have 13 pins between the punch marks as seen by the pics above so it looks like i need to move them closer together. The link also says I need to have the back to the exhaust cam punch marks to be lower. So maybe I should move the intake counter clock wise 1 tooth and make sure there are 12 pins between them and when the intake is lined up-ish at TDC the exhaust will be lower on the back?

Note...my tensioner is still tensioned in the pictures.

EDIT:Also I re-read your post william. It was not out of the blue; I had the bike apart for an extended period of time receiving new parts as mentioned earlier in this post. It ran well for an hour or so at a higher alt/cooler temp during the first ride. Then I fouled a plug. I replaced the plug and then the bike was driven for ~3 miles, parked, then 3 miles back where it then began to die and in a panic I leaned it up some and was able to sputter the last 1/4 mile or less back home. Since then it will die (after installing a new plug) after a few miles of riding, and with the fouled plug after it warms up.

ANOTHER EDIT: It appears my intake is adjusted correctly according to this picture and maybe I should only move the exhaust cam...then again he has 13 pins between the upper punch marks?


I don't think the 12 or 13 pins WR or YZ time matters those cams look like WR timing okay.

The 48 pilot is huge..i have a '00 400 with a 444 and I run a 42 in winter and 40 in summer.

I think you have an ignition problem.

I'd buy a coil and cap on ebay and swap and see.

Stator could be failing too.

Backfire means lean anyway.

Edited by R_Little

So you think the timing is fine as is?? I don't need 12 pins between the upper punch marks?

And isn't a 45 pilot stock?

And while I was under the general consensus that backfire was lean, most of the threads that turned up under a search said that a backfire was due to unburnt fuel get ignited in the exhaust, which also creates flames.

Additional input appreciated. I know everyone that has a wr/yz knows how to set up the cams etc...

I've got that single pop in the exhaust due to fuel in the pipe sometimes too.

Double check your valve clearance too.

Too tight valves will cause backfiring and stalling when the motor heats up and the valves tighten.

I think a 42 pilot is stock but the '98 has a slightly differnt carb than the '00

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