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CARBON

FOULED

REVERSED

COIL

POLARITY

ANALYZING USED SPARK PLUGS

NORM AL    A medium gray to light tan color

insulator plus normal electrode erosion ( less than .001" /    1000

miles) indicates good carburetor adjustment and correct spark pług performance.

Black, dry and sooty deposits in-dicate carburetor mixture is too rich. Check carburetor,air cleaner and autamatic choke.

OVERHEATED Light gray or vhite insulators with excessive electrode erosion and relatively free frcm deposit indicates mixture too lean and / or overadvanced ignition.

OILED    Wet, black deposits indicate piston

ring blow-by and oil by-passing valve guides. Check canpression.

A "cupped" side electrode with little wear on center electrode suggests possiblity of reversed coil polarity. Check for correct primary connections at the coil.