Sooty Plug

4 replies to this topic
  • yzrider02

Posted May 06, 2002 - 06:16 AM


I need an opinion on the way my plug is burning. I'll try to post a picture. I have a 2001 yz426 all stock except I leaned the needle 1 clip position from stock. This is a new bike with about 10 hours on it. I did the BK mod and disconnected the neutral switch (blue wire mod). The pump duration is set for about .75 sec. Fuel screw is at 1 3/4 and I'm running Sunoco 94 pump gas. The altitude is under 1000 ft. The bike runs good, starts good, but developes a black powdery soot deposit around the insulator and the spark plug threads. There is also a sooty ring around the electrode tip. This is a new Autolite 4303 (NGK CR8E Eqiv.) plug after about 4 hrs. of riding, all first & second gear on a MX track. Do you think the fuel screw needs to be leaned out or the needle lowered another position?
Posted Image
Posted Image

  • John_Lorenz

Posted May 06, 2002 - 07:07 AM


You are running a tad rich for sure, I am more concerned about the Crystal forming around the egde of the plug itself.

This may indicate additives in the gas or may be a after effect of the jetting..... Not Sure There.
I think this may be unburnt gases forming this way... Or an extreme lean condition in MID Range

What is your total Jetting Package

Main =
Pilot =
Air Screw turns out =
Needle position =

With your description of your riding habit, it may be a change in all areas.....

[ May 06, 2002: Message edited by: EgoAhole ]

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  • sirthumpalot

Posted May 06, 2002 - 07:09 AM


If it runs and starts good and isn't fouling plugs then why mess with it?

Also be aware that if you're allowing the motor to idle before you shut it off then your plug reading may not be accurate. You have to stop the motor while the throttle is at the setting that you want to check.

  • yzrider02

Posted May 06, 2002 - 08:07 AM


Main = 162
Pilot = 42
Air Scre = 1 3/4
Needle = third clip - 1 clip position leaner from stock

That isn't really crystal forming on the plug. It's from where the soot was scraped away. It just looks like that in the picture. I did foul a plug when starting the first time out, but I think that was from the fuel screw set too lean. It was only 1/2 turn out from the factory. Thats when I starting looking at the plug. I haven't fouled one since, but it looked sooty to me. It was idleing about 15 seconds before shutting off bike. Maybe this is normal. I just wanted to get some opinions.

  • John_Lorenz

Posted May 06, 2002 - 08:16 AM


Main = 162
Pilot = 42
Air Scre = 1 3/4
Needle = third clip - 1 clip position leaner from stock

If your idling the bike then the reading can be off even for 15 seconds.

The trick is to shut the bike off at the point you want the reading.

Meaning if you want a wide open read then find a straight tap the gears to fith WFO and pull in the Clucth and Hit the Kill button, cost to stop pull the plug.

Mid range is the same but under load say from corner to corner or ? but the kill is used the same.

If your bike is stock, only with BK and Blure Wire. I would not worry to much of the plug reading. Also note that the Autolite plug can very in the diff of hot cold if you are not using the NGK

Just for Craps and Grins get a NGK and run the same test to validate what the plug looks like.

Plug range in diff hot and cold catagories. You may have to jump one up in Hot for the Auto lite to. But first do a stare and compair

As shawn points out why mess with it if you have nor worries :)

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