READING PLUG COLOR


Plug color only indicates how the jetting is for the RPM range you use the most. For most people that is high RPM which means it indicates whether the main jet is too rich or too lean. That is very important but having that correct does not mean your mid throttle jetting is right, or that your jetting off of idle is right, or that your idle jetting is right. I highly recommend a good jetting calculator to know if the jetting is rich or lean between closed and fully open throttle.

Read the color on the first half of the ground electrode (not at its end) and end of the ceramic insulator. Medium gray or medium chocolate brown is the ideal color. This plug shows a slight richness, too cold a heat range, and too retarded ignition timing due to no clean band at the end of the center electrode. I was using Super M engine oil at 35:1.



I replaced that darkened plug with a new plug of the next higher heat range and with the ignition timing advanced and a smaller main jet. Here is its photo after a short run. The ground electrode doesn't show more cleanliness near its end. I was expecting the cleanliness with correct heat range but since it is mono-color that is probably due to the test run not being very long.


White colored ceramic insulators are caused by
1) too lean a fuel/air mixture
2) too hot a rating for the plug

Gray colored plugs can be caused by
1) use of aviation fuel
2) use of certain synthetic oils

Dark plug color can be caused by
1) excessive amounts of oil added to the gas (mostly with group 1 and 2 oils otherwise known as mineral oils. (Click here to see what group oils are in the most popular 2 stroke engine oils)
2) too rich jetting
3) spark plug misfiring possibly due to bad electrical connections or a failing ignition component (CDI, high voltage coil, stator coil)
4) too cold a heat rating for the plug

Be warned about lack of coloration at the side end of the center electrode that is more than a half millimeter. That is caused by too much spark advance or too hot a plug rating. Enough spark retardation allows all of the sides of the metal electrode except for the last half millimeter to have combustion deposits on it.


Cheap gasoline without enough deposit control additives can leave strange deposits on the plug. This is fairly common in 3rd world countries but also happens in 1st world countries. (see https://www.knowyourparts.com/technical-resources/engine/how-to-prevent-engine-deposits/)

How To Test The Main Jet
If you look at the dynometer graph below it shows the air-fuel ratio going rich before and after the meaty part of the powerband. So ideally you want to spend most of the time in that part of the powerband while testing the main jetting. You can ride on flat ground and keep upshifting and then in top gear apply the brakes to keep the engine from reving much past the meaty part of the powerband. Or you can ride up a long incline to keep the engine from reving past the sweet zone. If the main jet isn't too lean then you can get enough color on it to read it within one mile. If it is too lean then it may take 5 miles.



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