possible fouling fix

I bought a new 01 426 about 2 months ago, just as everyone else I began fouling plugs after break in. I spoke to a friend of mine who also bought one because he owns his own shop and he said his was doing the same thing. My bike even with a new plug would run like crap, and this is not my first 426 so I know how it should run. Anyway, my friend spoke to someone that he said is a Yamaha Technician, the guy told him to disconnect the light blue wire out of the cdi, he said it goes to the neutral switch and is causing low voltage output out of the cdi. I thought it sounded like a bunch of crap but I tried it, without any jetting changes and It ran like a dream today. I rode about 40 or 50 miles and it ripped. I had no problems starting it hot or cold, didnt even have to use the hotstart. I know it sounds too good to be true but its worth a try and its free. Let me know if you have any luck, so far Im extremely happy.

Norman K.

This sounds like it might work. I too foul plugs on cold days, and one of my friends (who rides a 01' 426) fouls plugs all the time this might make his day! It will be interesting to see what everyone says. I will try it tomorrow and post results! Later,


I just went out to the garage to find the light blue wire. It looks like you cant just disconnect the light blue wire, you must cut it? Am I looking at the right wire?

This is the wire I am talking about: If you find the CDI black box (located top right of steering stem) and follow the black strand of wires down about 7-8" you will see where all 6 wires plug in (mine is visible through the hole in the right radiator shroud) and they all plug in at once. This is the wire right? If so how did you disconnect it Norman? Thanks,


Garrett, unplug the harness and you will be able to see the inside of the plug, take a pick or a small needle and lift the plastic tab off of the connector, the wire will slide right out without any damage. I taped mine up and am gonna silicone the hole. I wanted to ride it again before I made anything permanent. Let me know your results, I hope this is the answer because I NEVER had any problems on my 2000 yz.

Norman K.

I too unhooked the blue wire . The bike started up without the choke . although it was about 60 degrees in the shop . I am optomistic though. I hope to have further posts down the road .As it was 2:30 am, and I havent ridden it yet. Tell me, is there anything bad gonna happen from unhooking the light blue wire. What is this wires main purpose . I wont have any problems elsewhere will I? :)

I will be unhooking the light blue wire later this week...and whenever I ride it next weekend I will post here on how it runs! I too was thinking this post would have had tons of replies. Will there be any side effects from disconnecting this wire, anyone? Or will it be OK like the WR's grey wire? Thanks,


Let us know how this works out. There are 3 of us here at work with the same bike/problems. What I don't understand though is, why is there a neutral switch on a YZ? I'm going home to peruse the schematics tonight but I'm kinda fuzzy on why this would work.


I seem to remember something about a neutral switch wire that changes the timing while in neutral for some reason or another. Try searching the message history. I'll bet you haven't done the BK mod. My 01 WR426 has wore out the front wheel bearings but is on the stock plug. It's YZ timed and has jetting similar to a stock 01 YZ. If you haven't done the BK mod it is easy to foul plugs while starting or warming the bike up due to the large amount of fuel that is squirted in when you twist the throttle. Just a thought, and is the only thing I can think of to explain why changing the neutral timing has improved the problem.

So if you start in the first gear with pulled clutch it's the same effect than unplug the cable,right ?...I'll try it out next time. My machine starts well in the moment but maybe there is a difference still. I don't think this will damage something because it's only the switch for neutral position...

Well I dont know if its just luck, or this really works . i unhooked the light blue wire three days ago . My bike has always be hard to start , almost impossible withought flooding it and having to install a clean plug first.Now the last two days I would pull the choke and kick it about three times and boom .keep in mind that I havent got the technique down pat yet ( I'm new to the 426 world ) But the temp. around hear gets down to 10 degrees or below at night . So in my eyes this has worked wonders. I was about to put my bike up for sale becouse I dont think a guy should have to spend about $6000 for a new bike and not be able to hop on and it fire right up . Now I have hope . I love this bike all over again . I just hope that it stays this way and it wasn't just luck . Right now its starting all the time .

Does anyone actually know what this neutral switch does? I looked in the manual and it tells how to check it, and how to replace it if necessary. The guy at my local Yamaha shop's service department didn't know what it was either.

Now granted, I'm not sure that he was actually a tech since this was close to closing time.

I hope this pans out, I'm getting the shi*s of replacing these $5 spark plugs.

Does anyone know how to clean the "fouled" plug and bring it back to life? I tried everthing, and finally got it to spark again, but the bike still wouldn't start until I put in a NEW plug.


I was fouling plugs all the time. Instead of buying new ones I took them down to the local Honda shop :) and they would glass bead them which is similar to snad blasting, only with glass beads. It came out very clean and the bike would fire right up. Without glass beading nothing would work. they usually did this for free, but charged a couple of bucks for 5 plugs at a time. Later.

I did my 2002 they said for 2002 they had some problems with it staying on causing it to run like the choke was on.I never fouled plugs on this bike,but now it starts 1or2 kicks no hot start needed.Someone on this site told me his buddy works for yamaha and said to do the mod. :)

[ March 12, 2002: Message edited by: h bomb ]

Man, Im glad to see it working for everyone else. I was hoping I wasnt having a lucky day. Lets see how it works out.

Norman K.

Started up again today . Second kick yeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeessssssss !!!! Anyway, its worked for me . have know idea what this blue wire is supposed to do, but what ever it is my bike starts easy and hasnt fouled a plug since .I used to foul a plug everytime I tried to start my bike . ( Everytime ) Now I will just have to hang on to my five spare plugs and hope I never need them . So far sooooooo good!!!!!!!!

[ March 12, 2002: Message edited by: rvanliere ]

Started up again today . Second kick yeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeessssssss !!!! Anyway, its worked for me . have know idea what this blue wire is supposed to do, but what ever it is my bike starts easy and hasnt fouled a plug since .I used to foul a plug everytime I tried to start my bike . ( Everytime ) Now I will just have to hang on to my five spare plugs and hope I never need them . So far sooooooo good!!!!!!!! :):D:D:D:D

Just got back from my harescramble race and the bike ran great, looks like this really did the trick.


someone posted once that he uses oven cleaner to clean fouled plugs. Sounds to me like it should work.

Ty davis makes a cover that replaces the wire harness coming out from the neutral switch. I may get it now that everyone seems to be having good luck removing the wire. Ty Davis would probably be a good person to call about the performance benefits of removing the wire.

ok, if that wire changes the timing or whatnot....and the rev limiter in neutral is lower than the limiter in gear, wouldn't it screw with your rev limiter, like move it up to the in gear limit. or maybe not have a neutral limit?

Man now i can't wait to take my bike back out its been in the shed all winter. I've been saving all the info that i get from u ppl.I wonder what it will be like to do the bk and this mod .Once the snow melts i will be busy suping up my bike. :):D

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