Spark plugs

Ignition system / Spark plugs

1 The correct functioning of the spark plugs is vital for the correct running and efficiency of the engine. It is essential that the plugs fitted are appropriate for the engine, and the suitable type is specified at the beginning of this chapter. If this type is used and the engine is in good condition, the spark plugs should not need attention between scheduled replacement intervals. Spark plug cleaning is rarely necessary and should not be attempted unless specialised equipment is available as damage can easily be caused to the firing ends.

2 At the specified intervals, the plugs should be renewed. The condition of the spark plug will also tell much about the overall condition of the engine.

3 If the insulator nose of the spark plug is clean and white, with no deposits, this is indicative of a weak mixture, or too hot a plug.

(A hot plug transfers heat away from the electrode slowly - a cold plug transfers it away quickly.)
4 If the tip of the insulator nose is covered with sooty black deposits, then this is indicative that the mixture is too rich. Should the plug be black and oily, then it is likely that the engine is fairly worn, as well as the mixture being too rich.

5 The spark plug gap is of considerable importance, as, if it is too large or too small the size of the spark and its efficiency will be seriously impaired. The spark plug gap should be set to the gap shown in the Specifications for the best results.

6 To set it, measure the gap with a feeler gauge, and then bend open, or close, the outer plug electrode until the correct gap is achieved. The centre electrode should never be bent as this may crack the insulation and cause plug failure, if nothing worse.

7 When fitting new plugs, check that the plug seats in the cylinder head are quite clean.

Refit the leads from the distributor in the correct firing order, which is 1-3-4-2; No 1 cylinder being the one nearest the flywheel housing (903 cc) or timing belt (1116 or 1301 cc). The distributor cap is marked with the HT lead numbers to avoid any confusion.

Simply connect the correctly numbered lead to its respective spark plug terminal (photo).

11.7 Distributor cap HT lead markings

Fig. 4.14 Spark plug connections on 903 cc engine (Sec 11)

Fig. 4.15 Spark plug connections on 1116 cc and 1301 cc engines (Sec 11)