Ignition fault finding

Clive Macintoshin Machinery & Mechanics

clive_mackintosh.jpgBasic Understanding: By Clive Macintosh.

When a petrol engine is turned over the flywheel is rotated round on the crankshaft. Attached to the inside of the flywheel is a magnet. On each revolution the magnet passes over the coil and produces a magnetic field. At this moment, the cam that is also attached to the crankshaft opens the points creating a spark between them.

This spark then passes back through the coil and up the HT lead to the spark plug.

This is termed the ignition timing system.

No spark at HT Lead

By holding the HT lead to the cowling and turning the engine over no spark can be seen from the end of the HT lead.

The lead should be flexible and free of splits. Over time HT leads can become hard and non-malleable with use and the lead may be broken inside. Also feel and check for any splits or breaks as the lead may be shorting out onto the engine through a split lead. In either case the lead needs to be replaced.

If the cable seems fine then the ignition timing system needs to be looked at.


To gain access to the points remove the cowling and the recoil starter on manual start machines.





Undo and remove the locking nut and the washer, normally a 'puller' is required to release the fly wheel but I can usually remove it by gently tapping the back of the rim with a hammer whilst turning it round.



Inspection of the points

The points produce the spark from the magnetic field created by the coil and the magnet in the flywheel.

As the flywheel turns, the magnet and the coil come into proximity once each revolution.

As this happens, the cam fitted to the crankshaft reaches its high point (lobe).

The moving part of the points is on a cantilever. At one end, pressed against the cam is the Bakalite heel, at the other the positive half of the points. The heel is fitted against the cam, so on each revolution of the crankshaft/flywheel, the 'lobe' on the cam pushes the heel out and opens the gap between the points.

As this occurs at the same moment as the magnetic field being produced, a spark is formed across the gap between the positive and earth points.

This spark then travels back through the coil to the spark plug, via the HT lead.


1) The points can become carbonised and pitted from the spark production.

Reface the point surfaces with emery cloth by lightly rubbing them until they are flat and shiny.


To check for the correct clearance turn the crankshaft so that the lobe on the cam coincides with the bakelite heel opening the points to their widest position.


Useful tip. If the feeler guage doesn't have the correct size then use a combination of smaller sizes to achieve the correct width.

Now close the gap on to the feeler guage by adjusting the eccentric cam screw located above the locking screw. Once this has been achieved, tighten the locking screw back up.


Earth the HT lead against the engine and turn the flywheel clockwise to see if a spark is now generated.

If there is still no spark then check the contacts of the leads from the points to the coil and from the HT lead to the coil.

If a visual check of the contacts seems fine, then replace the points and reset the points gap as before. Sets of points usually cost between £10-16.

In my next piece I will talk about the basic checks for fuel starvation.

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Machinery & mechanics