There are causes of a small dwell angle in an ignition system.

When fitting new contact-breaker points in the distributor, the normal way of checking the gap between them is with a feeler gauge.

Points that have been in use for a while develop a peak on one face that corresponds with a crater on the other, caused by spark erosion as the points open.A feeler gauge can't give an accurate measurement when this happens.

A dwell meter is used to measure the angle of rotation of the cam through which the points are opened and closed.

Without removing the distributor cap and with the engine running, it can be used to check the gap on worn points with more accuracy than a feeler gauge.When fitting new points, the meter can be used.

A dwell meter is an instrument used for mechanical checks.To be sold as an analyser or test meter, such instruments have to be switched to dwell for a reading.

When the distributor shaft is rotating, the contact-breaker points open as the heel of the moving point is pushed by the cam, and close as it is over the flat area between two lobes.

If the angle of rotation between the centre of the lobes on the cam is 90 degrees, the dwell angle - the period with the arm over the flats and the points closed - may be 52 degrees; the remaining 38 degrees are taken up by the action of opening and closing.This is a typical dwell angle for a four- cylinder engine.

While the engine is running, a dwell meter must be connected between the distributor and the earth to register the dwell angle on a scale.If the dwell angle is not the same for all cylinders, the result is rough running and poor fuel economy because the spark-plug fires vary from cylinder to cylinder.

Check the dwell angle in the service manual for the make of the car.There are scales on the meter for four- and six- cylinder engines.The vehicles are halved and taken from the four-cylinder scale.

Follow the instructions.If it is outside the distributor body or the CB terminal on the coil, one lead is connected to the side terminal.

The black and red leads to the distributor or coil in most modern cars.

The red and black leads of the meter should be connected to the distributor or coil on a vehicle with positive earth.

If you just fitted new contact-breaker points, make sure to set them to the right gap with a feeler gauge.

When the dwell meter is connected, start the engine and let it settle.If it doesn't tick over quickly, check the cause and fix any problems.

At a steady engine speed, the meter reading should stay steady.If it doesn't, increase the idling speed until it does.

You can compare the steady reading on the scale with the desired dwell angle for your car.

Make the following checks before you switch off the engine.If you want to increase the engine speed slowly, first ask a helpers to press the accelerator, then let it go back to idling speed while you note the reading.

If you notice the reading, increase the engine speed to about 1,500rpm, then let it go back to idling speed.

You can take an average of any differences in the readings if you carry out these two tests several times.

The distributor-shaft bearing or advance-retard plate may be worn if the reading fluctuates more than two or three degrees.Remove and refitting a distributor.

When the contact-breaker points are fully open, turn the engine by hand in its normal direction of rotation.

You can adjust the gap on some distributors if you loosen the contact-breaker fixings slightly.If the reading is too low, reduce it to increase the angle; if it's too high, increase it.

If the distributor cap is removed, replace it with a new one.Check the dwell-meter reading when you switch on the engine.If the figure isn't correct, switch off and reset the gap.