by Art Verlengiere
of R.L.V. Tuned Exhaust
Reprinted from Inside Karting Magazine
How can you tell if a plug is too hot, too cold, or "yes" just right? Let's assume that you have a general idea on the correct plug for your application, Take the vehicle out on the track, run it at race speed and race temperature for several laps. Then bring the car into the pits hot and shut it off-no idling. To read the plug, you need to look at both the insulator and the electrode. The table below was given to me by Gary Hartman many years ago. I don't know where he got it, or if he wrote it, but it has been very helpful to me.
Spark Plug Condition
Indications Normal - correct heat range Insulator nose white or very light tan to rust brown. Little or no cement boil where the center electrode protrudes through the insulator nose. The electrodes are not discolored or eroded. Too cold - use hotter plug Insulator nose dark gray or black. Steel plug shell covered with tar-like deposit. Too hot - use colder plug Insulator nose chalky white or may have satin sheen. Excessive cement boil where center electrode protrudes through the insulator nose. Cement may be white or meringue-like. Center electrode may 'blue' and be rounded off at the edges. Earth electrode may be badly eroded or have a molten appearance. Pre-ignition - use a colder plug Insulator nose blistered or fused. Center plug and remove piston and electrode and side electrode burned or combustion chamber deposits melted away. Detonation - retard timing Fractured insulator nose in sustained and/or richen mixture extreme cases. Insulator nose covered in tiny beads of aluminum leaving the piston. Excessive cement boil where center electrode protrudes through insulator nose. Specks on plug shell end.
Don't forget, only read the plug when it's new and has been run hot and brought into the pits and shut off - no idling. Good Luck and Have Fun!!
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