Plug Fouling Continues

OK TT Gurus!

I posted a few times a couple of months ago about my plug fouling problem and got some great ideas to try but nothing worked. Heres the skinny! My bike is a 1998 WR 400 used as a dual sporter. It is fairly low mileage and in excellent, clean, well maintained condition. The bike ran fine until early this summer when it started fouling spark plugs at least each ride. It continued to get worse until it now fouls a plug after about 2 minutes of running. I have replaced all electronic components including the CDI, Stator, Ignition coil and plug cap, Unhooked the grey wire, tried it witht he stop switch and TPS unhooked. Still no difference. I went torough the carb with a fine tooth comb and dropped the needle all the way. Checked float height and even ran it without the air filter to be sure it was not causing too much restriction. No Change. It fouls a $3 plug after only a few minutes of operation. Yes the plugs are black but I cant get enough time on the engine to take a proper plug reading. I believe they are sooting up while in the start-up mode. It runs great with no black smoke until it fouls the plug a few minutes later. Some of you said re-jet the carb but, it ran flawlesly for 3 years and probably on the same plug and all of a sudden started this. My dealer is no help, Yamaha customer service has been no help. This bike is un-ridable. If anyone has any ideas please help. I would buy a new 450 if I could get this one to run long enough to trade it in.

:confused:Sorry for being ignorant... how does a foul plug looks like... isn't they suppose to be black when used? :):D

Have you checked the compression? This would give you an indication of the contition of valves and rings. Also there is a Blue Wire mod for the newer WR's not sure if its the same for a 1998. There is info on this site that explains the Blue wire. This retards the timing while in neutral. Disconnecting it advances the timing and the bike burns cleaner. Also look at the Taffey mod. Good luck.

Sorry...I meant that I did the Blue wire mod. To my knowledge you can't do the grey wire disconnect on this year bike.

Thanks for the replys!



Have you had a Leak Down test done lately?

I am stumped.

If we rule out electrical, which seems prudent given you replaced just about everything, it must be that your carb is leaking fuel into the motor somehow. I think there is a gasket between the top and bottom halves of the FCR.

It sounds to me like you need to find a bike to swap carbs with. A KTM four stroke or DRZ should work as a donor if you can't find another 400/426.

If you swap carbs and the fouling goes away I'm thinking it must be the accelerator pump or a leak between venturi and carb bowl or emulsion tube or something.

Hope this helps. :)

Got a tough one here. I agree with Hick and would like to add maybe a bad float neeedle and seat or the float is filling with fuel and sinking, cracked slide????

Let us know.


Restriction on the exhaust? how about air mixture jets?

I have to think mabey in the float valve?

how about air mixture jets?

I dunno, I had a clogged pilot air and my bike would not even idle, and I gave it plenty of time to foul a plug before I finally yanked the carb.

What you and Bill say about needle valve etc. is a guess I'd also like to endorse as the culprit.

make your next $3 plug a lower number. this makes them more resistant to fouling. so if you have an 8 go for a 7 or even a 6.

next time you attempt to get it running, don't do it in the shed-do it at the beginning of the trail and get going immediately. even start it with your lid and gloves on the lot.

try and cover a few miles. disconnect the APJ if you can and you should burn a lot of the carbon off.

if the bike runs ok and but for what you'd call carb glitches it's fine then i suggest you drastically alter the jetting on it and come right down on your settings to the stuff they use here.

watch out for the coil wire being a bit loose as well...


Thanks for your posts! I will continue with some of the suggestions here and I will let you all know if and when it is cured. Taffy, what is the APJ?

what is the APJ?

He's wanting you to ride it with the accelerator pump disconnected.

Probably the easiest way to do this on your carb is to take off the pump cover (on the bottom of carb bowl), watch out for two very tiny o-ring seals, and remove the pump diaphragm and spring.

You can also remove the pump actuating rod, but I'm not sure how difficult that is to do on your carb, the throttle shaft may have to come off...

yes hick, whichever way it's done, that is waht i suggest!

what you need is running time with no problems, get some heat into the engine and burn all that carbon off. get fresh fuel through it etc etc.

then every little glitch needs putting down in your memory and briing it back here. we'll do our best.

now don't do what one bloke did and that is to find a bog in the acceleration "ah he said, it nearly jumped off the paddock stand as well!"

you have to smile


How many turns out is the fuel screw? Have you tried turning it in to lean the low speed jetting?

Is the choke circuit closing?

From your descriptions the jetting is nearly stock:

#45 pilot

?? turns out

#75 pilot air jet


#168 main

#200 main air jet

Is this correct? :)

It will get figured out for you. You got both James Dean and Taffy on it. :)

Thanks again guys!

I will try running it with the Acc Pump non functional. What if I try to ride it with the hot start valve pulled out. Wouldn't this lean it out and possibly eliminate rich fuel fouling the plug. The mixture screw is out 1.25 turns.

I will have some time this weekend to do some testing and I just bought a whole box of plugs.


The Hot Start is a good idea, playing with that in certain situations can tell you some things about your fuel mixture. For example, if a warmed up bike will still idle with the hot start pulled, it is probably too rich at idle.

I hope you get it figured out. I don't want to see another unresolved plug fouling issue here...

to disconnect the APJ you can loosen the small bolt/nut that doubles as the pivot. This gives you some play to gently flex the black plastic bellcrank and pop the rod right out.


I am still experimenting with this gremlin. I turned the pilot screw in to 1/4 turn out. It was set at 1 and 1/2 out before. It does not idle real well this far in but it does seem to be leaner. I ran it about 20 minutes around the yard and it has not fouled yet. Usually the second time I crank after it has had time to cool completely the plug will foul for sure. I did stop and check the plug a few times during my yard riding and most times it looked good and clean. However, a couple of times it looked good in the center but the outside rim of the plug was black. It would easily wipe off with a rag and some carb cleaner. Is this normal?? I don't put the bolts back in the gas tank or seat so it is easy and quick to pull the plug. I have not disabled the Accelerator pump yet. I am trying to do this one step at a time. I will continue to try it this week. Thanks for all the help and suggestions.


I'd have to agree with Bill, check your float needle, this happened to me on an older cr250. Not only did the float valve have a ring around the cone, but the seat in the carb itself was worn, not allowing the valve to properly seat. Just a thought.

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