Conditions of spark plugs

NORMAL APPEARANCE

The insulator nose is covered with thin, light brown or grayish deposits. Spark plug heat range is correct and all engine systems are properly tuned. Replacement of spark plugs should be done after 15.000 kilometers.

THICK DEPOSITS

Thick deposits on a spark plug (as shown) may be caused by the following:
- too much lubricating agent
- impurities in fuel or oil
- worn out valve
guides Spark plugs should be cleaned.

SOOTED SPARK PLUG

Carbon (soot) deposits on the insulator nose and plug body may cause short circuit between the electrodes what makes a spark weak or extinguished. This condition may be caused by: - over - rich mixture - air damper (choke) mechanisam malfunction

OILED SPARK PLUG

Insulator is covered with black and wet oil deposits which may over . bridge the gap resulting in a weak or extinguished spark. This condition may be caused by:
- worn valve guides
- worn cylinder bores, pistons and rings
- to much oil in petrol
- oil mixture in two-stroke engines
- over - filling the sump with oil
The problem of oiling should be solved.

OVERHEATING

The insulator nose is dry and clean, covered with bubbles. Electrode errosion is more pronounced. This condition may be caused by:
- lean mixture
- ignition distributor malfunctioning
- fuel with low octane number
- advanced ignition
Overheating cause should be eliminated and such spark plugs replaced.
- Too "hot" spark plug fitted.
This is eliminated by fitting a spark plug of edequate heat range
(Exp. FE 65 P colder FE 85 P)

PREIGNITION

Results from high overheating. The causes are the same as with overheating, but more emphasized. It is neccessary to eliminate the preignition cause immediately to avoid any damages to the engine. Such spark plugs should be immediately replaced by "colder" ones.

CRACKED INSULATOR NOSE

Thin line on the insulator nose is an indication that cracking will occur. This is mostly caused by engine detonations which indicate the following:
- advanced ignition
- improper operation of the ignition distributor
- low octane fuel
- lean mixture incorrectly adjusted gap These causes should be eliminated and spark plugs replaced.

LEAD DEPOSITS - LEAD GLAZE

This results from lead impurities in the fuel. The deposits my cause misfiring.
Fuel quality (lead impurities) should be checked. After long intervals of slow drive, the speed should be gradually increased to enable the engine to clean the deposits away. It is not required to change the spark plug type.