Overheating
The insulator is dull white or gray and appears blistered. The electrodes are eroded and there is an absence of deposits.
Ensure that the correct plug heat range is being used. Also check for incorrect rotor location, cooling system malfunction, lean
fuel and air mixtures, leaking intake manifold, or sticking valves. Replace the spark plugs.
27648
High Speed Glazing
The insulator has yellowish, varnish‑like color, indicating that the temperatures suddenly have risen, usually during hard, fast
acceleration under heavy load. Normal deposits do not get a chance to blow off. Instead, they melt and form a conductive
coating. Replace the spark plugs. If the condition recurs, use colder heat range plug and service plugs more frequently.
27649
Scavenger Deposits
Powdery white or yellow deposits are built up on the shell, the insulator, and the electrodes. This is the normal appearance with
certain branded fuels. Accumulation on the ground electrodes and the shell areas may be unusually heavy, but may be easily
chipped off. Service (clean, file, and regap) plugs and reinstall.
27650
Engine Troubleshooting
Page 3C-6 © 2016 Mercury Marine 90-8M0099748 eng DECEMBER 2015
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