spark plug problemes

My YZF426 is fouling plugs pretty bad used to be every couple of ride now it's every 20 to 30 minutes. I brought to the dealer and they checker valve clearence and cleaned the carb and checked the jets and say everything is good they say it might be the way I start it but pretty sure its not

What could it be? The plugs are all black and dry

Sounds like a lean fouling condition, may need to richen it up a bit. I'm not the expert on the board, others with a better idea will chim in I'm sure. But, give us some details. How's your jetting??? Throw us some carb spec's, Main Jet, Pilot Jet, Pilot Screw position, Needle positon, Elevation, Humidity, temp and what not. There are some supper sharp riders here, they'll get you up and rolling.


Dodger :D:)

You may want to first try the blue wire mod, some of he guys that had plug fouling issues say it helps alot.

Some also found that the CDI unit itself was bad to begin with. If it started out as a once in awhile thing now its every 20 minutes, you may need to see if Yamaha will replace the CDI..

Others will answer Im sure

how much does a cdi unit cost?

Turn the fuel screw in the bottom of the carb in 3/4 of a turn. If it gets better, get a smaller pilot jet. If its the same, start lookin harder at the CDI. Yamaha is warrantying some of this stuff, but you cant just stomp into the dealer and demand things. Ask him nicely if he'll check into the CDI box for ya, because you heard of about 40 guys with the same problem.

On a side note, your plug shouldnt get any darker than a really boring beige.

Also on a side note I forgot to add,,, What Gas are you burning are you sure you have not got a bad batch?

Just asking but you should be runing at least 91 octain or Higher. And a Bad batch of Gas can be an issue.???

I've got the same problem, did the blue wite mod, no problems, the a couple of weeks ago I fouled two in two days, missed practice, streesed out and went over the bars. Plug is jet black. Carb settings are what they were when I got it.

four strokes shouldn't foul plugs unless your crashing many times do you fall before it fouls?humidity also can be a factor.i noticed it on my yz400...... isn't it fun changing the plug? whoever designed where the plug goes should be smacked real hard.takes me 30 mins to change a freekin real fun when the bike is blazing hot too.

First of all what year Yzf? Does the BK Mod sound familiar? It worked for me and a whole lot of other people on this site. Do search on it there are 100s of posts. Good Luck :)

My '01 was fouling like crazy (one per ride, always on the first start of the day) and here's what fixed it for me. I don't use Mobil brand gas anymore and now no fouling problems at all! I've also noticed that if I start it when it's cold and let it idle for a few minutes and then turn it off and leave it until the next practice (days) that this will sometimes kill a plug. If I take a lap around the yard before turning it off then it won't foul. However this also happens on my wifes stock '01 XR100 so it must be the gas in our area?

I have a three month old YZ426F and mine did the same thing, really hard to start and black sooty plugs...I ride in Colorado and the manual says to use a #158 main...well S@@T can that, I went to sea level #168 and my problems were I have a stripped oil filter drain bolt!

I am not sure but I was going to get a 2001 yzf and the guy said it kept fouling because of the coil? Well I did not buy it I bought a 2002 and had it for 8mts and had no problems.


Do what ego says about the blue wire. A bunch of us '01 owners did it with good results.

Do a search on blue wire mod and you should see a lot of posts from people who tried it. Worked for me.

Did I just say do what ego says? Yikes


[ August 16, 2002: Message edited by: Robertski ]

The CDI box is $224.82

Do the blue wire mod. Run fresh gas (I use 76 race gas). Adjust your idle fuel mixture and the problem will be gone.

Skin has the best suggestion. Id change out the gas first. Then start messing with the settings if things dont improve. Just try a different brand of fuel. Some sort of race fuel would be best. VP C12, or as stated earlier 76 100 octane unleaded works very well too. Its what Ive been running for quite a while now with postive results.

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