I have a theory on all this plug fouling...

First of all, many thanks to James and Clark for your suggestions to my WR starting/fouling woes. I have a 98 WR400 with WB E series, tapered head pipe, air box top removed, YZ timing. I was running the DTM #4, 45PJ, 2 turns, 162MJ and I was regularly fouling plugs. I have now gone to the DVP#4, 48PJ, 1.25 turns, 172MJ and it seems to run great. I still want to try a #5 clip with a larger MJ. Thanks Clark and James for your help, I'm finally enjoying my ride!!!

Anyway, here is my theory...

I've noticed I would foul plugs shortly after starting the engine. The last time it happened I just got all suited up and started to head out and on came the snarling and poping. My theory is these carbs shoot an unbelievable amount of fuel once the throttle is opened, and could it be this stream of fuel are heading into a cold cylinder and plug with no hope of vaporizing. I was quite impressed with the distance that stream shot when I had my carb off to do my changes.

So, lately I have been going with a new procedure by starting it and letting it idle just long enough on choke until it will idle on its own with the choke off. I then let it sit and idle until the cylinder head is quite warm to the touch and only then get into the accelerator pump. Maybe we're getting a bit anxious with these "squirter" carbs (never had one before!)

So far, no fouled plugs! Any thoughts???

I wonder if it could be the colder weather we are experiencing requiring a little longer warm up period. You might be on to something there.

When you pull out the (fouled) plug is it fouled or just wet with gas. Another thing is that the new oxegenated fuel promotes the condensing of water in fuel that has sit around for any length of time. Which ends up at the very bottom of your gas tank/float bowl. A way to check for water in your fuel is to take a clear glass jar and empty a small amount into the jar via the petcock, the gas should be on top with any water in the bottom. Ask a pilot as in aviation, as this is one of their preflight tech inspection rituals. I guess if you run race gas this option could be deleted.

gassed, huge


I aggree with all of your ideas - getting the choke off early, warming it up good, and not over-twisting to warm it up which squirts raw gas in.

Your switch from DTM#4 to DVP#4 has broad effects some readers may not realize. DVP#4 is larger and leaner all accross the range and is an appropriate size step to make. Thank Clark for that one.


I agree the constant bleeping of the throttle is shooting a fresh stream of fuel directly into the intake everytime the throttle is opened. I'm starting to learn to limit the amount of opening and closing of the throttle until the bike is good and warmed up. I still don't understand why my plug comes out black when running the DVP in the leanest position after a long enduro while others are running in #4 position with no problems. There must be variations in the fuel pump output or ignitions of various WR's. "I am a plug fouler" there admitting it is half the battle... :)


Measure your carb needle to confirm your sanity :D .

Get some calipers and measure your DVP needle diameter, the straight portion should be 2.735mm (.1077in). Then set the calipers to 2.50mm and hold it horizontal. Compare DVP and your stock needle together by letting them rest gently in the slot. Look at the relative clip height.

A stock '99 DTM needle will rest EXACTLY 1 clip position further down than DVP.

A stock '00 DRS needle will rest EXACTLY 2 clips down, it is 2 clips richer at mid throttle.

If your needle rests 5 clips lower, it is actually a DKP needle!! and is mis-marked. This would force you to the leanest possible clip position.

--We can hold weekly meetings for those interested in "jetters anonymous". ... Hi I'm James, and haven't fouled a plug in 5 weeks... :) .... Now I can enjoy riding again. :D


ive read alot on fouled plugs on this message board and others , and nobody seams to mention what plug they're running? a flooded engine will wet the plug and it should dry out and fire again unlike a 2 smoke that will oil foul a plug and your not going anywere, on my 01yz426 i was having lots of trouble after a stall restarting i pulled the plug and it was an "nd", when i bought my xr400 a few years ago it also had an "nd" and when i got it home i couldnt make the thing run! i towed it with a 4 wheeler and it still wouldnt fire, i changed to an "ngk" and off she went as an experiment i changed back to the "nd" same thing it wouldnt start to save its kickstarter! i have since changed to an ngk on my 426 and what a difference!

thanks to clark i changed to a 48 pilot 3/4-1 turn out and now she starts much better and stalling is almost gone completely, i noticed that the nipondenso plugs dont seam to self clean as well as the ngk's after being flooded just my 2 cents worth.....

To answer Howard, I think it is just wet with gas when it fouls. That begs another question: do you think this stockpile of CR8E plugs I've amassed can be reused? I've always not wanted to temp fate and slapped a new plug in. Sandblasting recommended before reuse? Also, what about going to a hotter heat range? I'm too scared of making a very expensive short skirted ash tray to give that one a try.

Then again, maybe I won't have to worry about our favorite topic anymore. One can hope!


Wet plugs can be dried out without removing them from the engine. All you need to do is to get air to the plug. Which will evaporate the fuel. Viola dry plug. I don't have a problem with my 400, but my dads klx 300 does this regularily. The process for drying out a wet plug is to hold the throttle wide open and hold it there then kick the bike over you ,can do this with the compression release pulled in about ten or so times. Then proceed to start your bike all the while not letting off the gas! The gas part is important due to the accelerator pump. Spark plugs are a pain to change on these things so give it a try. Works like a charm on my dads thumper, motor is a motor right? You can reuse your wet fouled plugs once they dry out. .02 cents keep the change, huge

James thanks for the recommendation to measure my DVP needle. I will do this next time I need to get at my needle. I did experience the warm orange glow of the headpipe last night. Part of this could be due to the temps dropping 40 degrees F. I am not sure if it did this when the temps were warmer, but I do feel reasonably confident that my pilot circuit was not causing the black sooty fouling plugs. Thanks again for your advice so far!


2000 Yamaha WR400F

1988 Honda NT650 Hawk

1977 Suzuki RM250B

1974 Honda XR75K1

NGK CR8EK. For the life of me, I can't seem to comprehend why some owners are experiencing the fouled and wet plugs. I installed a new donut gasket at the head pipe/"S" bend juncture and noticed the inside of my head pipe was a nice tan color....same for the spark plug. I will be glad when this phenomenom comes to fruition, but not as much as the owners who have the problem. Curiosity is killing me!


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