Spark plug help

Anatomy of a spark plug

A spark plug basically consists of 5 components:

  • The terminal allows electrical connection from the plug wire to the spark plug.
  • The ceramic insulator isolates the terminal and center electrode ends from the ground electrode and houses the resistive element of the plug.
  • The body of the spark plug consists of the threads for installing the plug as well as a hex for turning the plug with a wrench.
  • The center electrode provides the source point of the spark.
  • The ground electrode provides the destination point for the spark.

How a spark plug works

The engine ignition control circuit determines when the spark is to occur. At this time, a very high voltage pulse is delivered to the spark plug terminal end. The voltage is so high that electrical current can flow across the gap between the center electrode and the ground electrode, resulting in a spark. Of course, it is this spark which ignites the fuel-air mixture that has been drawn into and compressed in the engine combustion chamber. However simple a spark plug may be, the construction is critical. A spark plug must be able to withstand frequent high voltage pulses, high pressures, high temperatures, vibration and combustion by-products. Each of these variables contribute to the possible failure of a spark plug.

How a spark plug fails

A spark plug can fail in three basic ways. 1) mechanical failure, 2) fouling, or 3) overheating.

Mechanical failure:

Mechanincal failure of a plug would likely be a broken insulator, stripped threads, damaged sealing surface or sealing ring, or a broken terminal end. These failures are likely to occur from poor handling or careless installation and are usually obvious to visual inspection. While possible, mechanical failures are not likely occur when installed correctly.

Fouling:

A spark plug can fail by what is referred to as fouling. Spark plug fouling occurs when the insulator nose at the center electrode accumulates either fuel, oil or carbon. This accumulation allows the high voltage to short circuit along the insulator nose to ground rather than jumping the electrode gap. A fouled spark plug can sometimes be cleaned to restore normal operation. Often, however, replacement is the only solution.

Possible causes of fouling:

  • Continuous low speed engine operation
  • Spark plug "heat range" too cold (the selected spark plug is incorrect and cannot reach the correct operating temperature).
  • Air-fuel mixture too rich resulting in accumulation of fuel deposits around the insulator nose and electrode.
  • Reduced compression and oil usage due to worn piston rings, worn cylinder or worn valve guides.
  • Excessive premix oil in fuel for two stroke engines.
  • Retarded ignition timing.
  • Ignition system not delivering the correct voltage.
  • Spark plug gapped incorrectly.
  • Dirt, moisture or other conductive contamination in and around the spark plug terminal connection.
  • Failed spark plug wire.
  • Dirt, moisture or other conductive contamination in and around the magneto/alternator.
  • Loose, broken or contaminated ignition system wiring connections.

Over heating:

A spark plug can fail by overheating, which can cause the electrodes to wear quickly, melt or break apart. The failure mode of the electrode depends in part on the type of electrode material. Engine overheating has an array of causes and harmful effects not limited to a spark plug.

Possible causes of over heating:

  • Incorrect spark plug heat range (the selected spark plug is the wrong type and cannot dissipate the correct amount of heat).
  • Spark plug not tightened to the correct torque or the gasket is missing.
  • advanced ignition timing.
  • Fuel octane rating too low or excessive engine compression due to excessive combustion chamber deposits (engine will knock).
  • Air-fuel mixture too lean.
  • Continuous engine operation under excessive loads.
  • Insufficient engine cooling or lubrication.

Visual examples of failed spark plugs, courtesy of NGK:

Normal

Insulator Breakage

With Deposits

Dry Fouled

Electrode Erosion

Lead Fouled

Electrode Melting

Overheating

Oxidation

Wet Fouled

How to test spark plugs

It is possible for a spark plug to fail yet appear normal. Using an ohm meter, you can check a suspected bad plug.

1 Connect one test lead to the spark plug terminal and the other lead to the center electrode (note: you may need to clean the center electrode to have good continuity with the meter lead). If you have a resistor type spark plug, you should have a reading of several thousand ohms (around 5k ohms, plus or minus 1k ohm or so). If you have a non-resistor type spark plug, the meter should display very low ohms or zero. If this measurement shows an open circuit condition, the plug should be replaced. If the measurement indicates a good spark plug, proceed to the next test.

Click below for graphic view:

2 Leave the test lead on the spark plug terminal end and move the other lead the ground electrode or any place on the metallic body, below the ceramic insulator. It is important that you keep at least one hand from touching the plug or test lead tip to avoid a false reading. This measurement should show an open circuit measurement. Any measured resistance (even in the meg ohm range) is bad and will cause the high voltage pulse to bypass the high resistance air gap, resulting in no spark or weak spark.

Click below for graphic view:

Please consult manufacturer technical specifications for any given spark plug.

PARTS AND SUPPLIES

Need parts and supplies for your dirt bike? We have you covered with all the parts and accessories you need from aftermarket to OEM.

ENGINE AND TRANSMISSION PARTS AND KITS

SHOP NOW

FUEL SYSTEM COMPONENTS

SHOP NOW

IGNITION COMPONENTS

SHOP NOW

EXHAUST SYSTEMS AND PARTS

SHOP NOW

INTAKE PARTS AND KITS

SHOP NOW

ENGINE OILS AND COOLANT

SHOP NOW

ENGINE TOOLS

SHOP NOW

OEM PARTS

SHOP NOW