Spark plug reading — interpreting black sooty deposits When a spark plug shows a dry, black, sooty coating on its insulator and electrodes, what does this typically indicate about mixture strength or combustion?

Introduction / Context:
Spark plug inspection is a quick diagnostic for air–fuel mixture and ignition health. Colour and texture of deposits reflect combustion quality and mixture richness.

Given Data / Assumptions:

• Observation: black, sooty, powder-like deposits.
• Operating on gasoline spark ignition engine.
• No oil fouling symptoms (wet, oily, shiny deposits) reported.

Concept / Approach:
A rich mixture supplies excess fuel relative to available oxygen, causing incomplete combustion and carbon soot formation. The soot appears as a dry black film. Lean mixtures tend to cause white or light grey deposits and can raise combustion temperature, risking knock or pre-ignition.

Step-by-Step Solution:
Identify deposit type: dry black soot.Relate to mixture condition: over-fuelling leads to sooting.Choose the option indicating a too rich mixture.

Verification / Alternative check:
Tuning guides show coffee-brown/light tan as ideal, black soot as rich, and white blistered insulators as lean/overheated.

Why Other Options Are Wrong:

• Too lean or stoichiometric: would not leave heavy soot; may show pale deposits.
• Most economical mixture: typically slightly lean of stoichiometric, again not sooty.

Common Pitfalls:
Confusing wet, oily black (oil burning) with dry soot (rich mixture). Oil fouling indicates mechanical issues such as worn guides or rings.

Final Answer:
too rich mixture