Example:

off. Faults can be traced to the cooling system or improper ignition timing.

To single out a faulty plug in the engine, proceed as follows:

• Remove the plug from the engine and push the plug cable connector over the plug.
• Place the plug on the cylinder head cover, ensuring that it cannot fall off when the starter motor is operated. An insulated pair of pliers can be used to hold the plug.
• Have a second person turn the starter switch and observe if there is a spark between the two electrodes. If a spark is visible, this is by no means an indication that the plug is in perfect condition, as it is possible that the spark cannot be produced under the compression of the engine. This can only be tested with a spark plug tester.
• If no spark is visible after several attempts, examine the plug. If the engine has been running with one of the plugs out of action this will be indicated by the fact that the spark plug will be saturated with petrol or oil and this is your confirmation that this particular cylinder has not been firing. This could be so for various reasons, with the extreme case that the piston rings are allowing oil to pass from the cylinder into the combustion chamber. There are, however, a few points which might be cured on the spot. The first remedy is, of course, to substitute the suspect plug with one known to be working to ensure that the plug is at fault. To do this, proceed as follows (let us assume that the No. 3 cylinder is the faulty one):

Unscrew one of the remaining plugs and check the appearance of the plug face. This will be dry and will most probably have a light brown colour. Screw this plug into the cylinder where the first plug has been removed and fit the faulty plug to the remaining cylinder. Re-connect the high-tension leads to the spark plugs and re-start the engine.

Check the firing of the engine by pulling off the H.T. leads between the ignition distributor and the plugs. If the suspected plug is faulty the engine should still run on three cylinders with the difference that the cylinder that was originally misfiring is now operating and that the good cylinder, where the suspected plug is fitted, is now out of action.

Example:

As already mentioned, it was assumed that the No. 3 cylinder (and thereby the spark plug) was faulty. Remove either Nos. 1, 2 or 4 spark plugs from the engine which should operate normally. For our example let us remove No. 1 spark plug.

Fit the No. 1 spark plug into the No. 3 cylinder and the No. 3 spark plug into the No. 1 cylinder. After the engine has been re-started it should now run on cylinders Nos. 2, 3 and 4 as the suspected plug is in the No. 1 cylinder this cylinder will, therefore, not be able to fire. This can be further confirmed by unscrewing the plug from the No. 3 cylinder which will now have a wet appearance, although it will not be the same as the plug face of the faulty plug because it has not been running for very long.

Remove the faulty plug (which is now in the No. 1 cylinder according to our example) and fit a new plug. It is still possible to rectify the fault if either of the two following cases apply:

1. The spark plug electrodes are too close together so that no gap exists.