Spark plugs with soot or contaminated before testing, clean on a special installation with a jet of sand and blow with compressed air. If the soot is light brown in color, then it can not be removed, since it appears on a serviceable engine and does not interfere with the operation of the ignition system.
After cleaning, inspect the candles and adjust the gap between the electrodes. If the spark plug insulator is chipped, cracked, or the side electrode weld is damaged, replace the spark plug.
Gap (1-1.1mm) between the electrodes of the candle, check with a round wire probe. It is impossible to check the gap with a flat probe, since this does not take into account the recess on the side electrode, which is formed during the operation of the candle. Adjust the gap by bending only the side electrode of the spark plug.
Leak test
Screw the candle into the appropriate socket on the stand and tighten with a torque wrench to a torque of 30.7-39 Nm (3.1-4 kgcm). Create a pressure of 2 MPa in the stand chamber (20 kgf/cm2).
Drip a few drops of oil or kerosene from the oil dish onto the candle; if the tightness is broken, then air bubbles will come out, usually between the insulator and the metal body of the candle.
Electrical test
Screw the candle into the socket on the stand and tighten to the torque indicated above. Adjust the gap between the spark gap electrodes to 12 mm, which corresponds to a voltage of 18 kV, and then create a pressure of 0.6 MPa with a pump (6 kgf/cm2).
Install the tip of the high voltage wire on the spark plug and apply high voltage pulses to it.
If a full-fledged spark is observed in the eyepiece of the stand, then the candle is considered excellent.
If sparking occurs between the spark gap electrodes, then the pressure in the device should be lowered and the pressure at which sparking occurs between the spark plug electrodes should be checked. If it starts at a pressure below 0.3 MPa (3 kgf/cm2), then the spark plug is defective.
Several sparks are allowed on the arrester; if there is no sparking on the spark plug and on the spark gap, then it must be assumed that there are cracks on the spark plug insulator and that the discharge occurs inside, between the mass and the electrodes. Such a candle is rejected.