Quote:
Originally Posted by jimbo1593
Slightly off topic....but this Q reminded me of the Haynes manual page with the COLOUR pics of spark plugs.
This was my favourite page as a boy...yes, i was and remain, odd.
Isn't red dusty OK, or maybe under fuelled a tad?
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The dusty red is typically a result of additives in the fuel or contamination of the fuel. If the red borders on a rusty red in can indicate a contamination with iron oxide (rust).
For fuel mixture, the color of the actual insulator is the tell. Sooty black to very dark brown would be over fueling. Light brown to bordering on white would be 'in the money'. Modern EFI with O2 feedback typically run right at an AFR of 14.7 and their insulators tend to be white. Carb equipped cars lack the corrective feedback so you typically play it safe and run with a mixture that gives you a light brown insulator nose. Chronic under fueling results in a bone white insulator with signs of what looks like glazed sugar on the insulator.
In this case the residues are on the external electrode and around the metal plug body so likely the result on unplanned or intentional stuff in the fuel.