426 plug fouling

I have a friend in WA that has an 01' YZ426, and he has a bad problem with plug fouling. Same symptoms as "arraCODEBLUE" Me being a DRZ rider, I'm not too familiar w/ the 426 peculiarities. Please let me know what you did to cure the plug fouling. He has completely stock jetting, but he installed the FMF "Q" pipe. I'm almost certain that the new pipe has choked up the breathing a little making him run rich. Does the 426 come from the factory already jetted rich? Elevation where he lives is approx 2,000 feet. He's pretty much non-mechanically inclined. He knows to be careful with the throttle because of the APJ.

If you guys concur, this is what I'll recommend to him:

1.adjust pilot screw to determine if pilot jet size is appropriate(I'll explain it to him)

2. drop needle one notch

3. install new plug

4. check float valve for proper seating

I believe the bike should be able to be tuned to not foul plugs without the BK mod, so I won't go that far with him......yet. Thanks guys! I promise I won't bother you often :)

Colorado- do a search. This came up several times earlier in the year but i don't remember the solutions. Do a search here and on the WR side.

The 01's have a problem with the accelerator pump squirt duration and timing. What is common is the duration is to long which sprays too much fuel into the head. This will cause the engine to "flood" which will then foul the plug.

--What you'll have to do is remove the carb and check those two issues.

--Keep the fuel line attached of at least make sure the float bowl is full.

--Point the cylinder side of the carb away from you.

--Twist the throttle as fast as you can to full stop.

--Time the duration of the pump spray. It should be around .50-.75 seconds. Mine was almost three and a half seconds.

--Check that the pump spray just misses the slide as it raises.

--Use the adjustment screws on the right side (as if you were sitting on the bike) of the carb to fine tune the duration and timing.

--There may be more you need to do to get the duration correct. This requires some drilling and tapping of the tab adjacent to the pump wheel on the outside.

--If the simple adjustments don't work, private mail me and I'll get you headed for the other adjustment. Don't fret, it is alot easier than it sounds!!!


There are lots of threads here about plug fouling and it seems that for different bikes there are different solutions. My '01 is completely stock, even the accelerator pump and I have no problems. I did at one time but for me the solution was switching brands of gas. I would search here and find all of the potential solutions and then try them one at a time starting with the easiest to try. Hope this helps. :)

I have the same problem with fouled plugs but only when I ride slow for a long time with my wife....if i ride with the hammer heads i have no problems.

my thought is the bk mod is needed (or just ride fast all the time!) :)

Many of us here in CO have this problem in the winter. My 01YZ426 had it bad. Using a CR7E plug helps some, but the real culprit on my bike is the ethanol they put in the gas here during the winter for pollution control. If I get gas from Highway 52 and I 25, I have no problems. They advertise "real gas" and they are in a county that does not have the ethanol mandate. If I buy gas near my house I will foul plugs and my bike will run terrible.

Most people here have had little luck by changing the jetting. I jetted mine ridiculously lean and it still fouled. My bike had the BK mod and that made no difference. Have him switch gas, and if he is not sure whats in it, have him try some race gas.

My '02 fouled every time I rode it for the first coupla weeks. (I live in Northern CO). All I did was switch the main jet from a 162 to a 158 and turned the pilot screw 1/4 turn. Ridden about 7-8 times since and the plug has been fine. A good indication of a jetting prob (besides bad plugs) is the needed use of the choke/hot start knobs. When Jetted correctly, you should have to use both of these to start the cold/hot bike. If you don't, chances are the jets are wrong.

Is there any real truth to CDI fix? "Different solutions for different owners/bikes"; Ya I realized that when I read a post "fouled plug=foul mouth" or something like that. Took me damn near 2 hrs to read the thread and it was confusing as hell! :) For some it was the CDI, for others changing gas, and the list goes on and on......... Has anyone come across a faulty float needle valve yet?

Thanks for all the input guys! My friend and I really appreciate it. :) I talked to him last night and he said as long as he turns the fuel off and lets it run for about a minute or so, there's no problems when he goes to start it again. He says that's a small thing to deal with as long as it keeps working. He put one of those Acerbis DHH side-by-side headlights on it and now he has a complete White Brothers fly-wheel(10oz heavier), and a Moose lighting coil on order. Should make for a decent woods bike.....don't ya think?

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