Jetting With Spark Plugs

          If anyone has read Gordon Jennings' article on spark plugs, it is a very good read. He mentions checking the mixture ring down at the base of the insulator for checking your main jet.

          In my own experiences, I see a lot of riders telling other riders that if the insulator on the spark plug is white, then you are running too lean. They do not bother to ask if it is a new plug or used plug. The reason I say new or used is, you will not color the insulator up by the tip if the plug is new. It takes some time to color that area of the plug. You can be running 5 sizes too rich, and you will still not get color up by the insulator tip. If you listen and feel your motor, it will tell you everything you need to know, but you have to put in the time. I do not like to jet by the plug color, there are too many variables involved. Different oil and fuel properties = DIFFERENT COLOR READINGS.

         Gordon Jennings talks about importance of looking at the fuel mixture ring down at the base of the insulator, and he is correct. I will prove his article is right on the money in the plug pictures I have taken on my 03 KX250. The only thing I did not agree with is how the ring is suppose to climb higher the richer you go. I found out that the richer you go, the darker the ring becomes not higher. I warmed up the motor with my plug I use at normal riding conditions, BR8EIX. I shut it down and put in a brand new plug.  As soon as the motor was running, I went off and did 3 fast passes at full throttle.  It is important to stay at full throttle when checking for the mixture ring (no idling or soft riding).  I held this for 7 seconds, then repeated the process with a new 2nd plug. I had my jetting all sorted out weeks before this test. Like I said, I like to jet by trial and error, but I was curious to see these results. The picture on the top is with a 148 main. This is what I chose just by listening and feeling the motor. I got her right on the mark, and the plug's fuel mixture ring does reflect that. The plug on the bottom was run with a 152, just to see if the mixture ring did change. As you can clearly see, the insulator tip on both plugs is still white.  However, if you look at the fuel mixture ring at the base of the insulator, you will see there is no risk of running lean on either plug.  This would only be a concern if the fuel mixture ring was barely visible or absent of color. If you are jetting by the plug color, you are wasting your time looking at the insulator tip of a new plug. I do believe that the best way to jet is by "feel" and listening to your motor.  However, if in doubt, this method is a useful tool.

-Keith LoCastro

148 Main

152 Main