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 some time develop a peak on one face that corresponds with a crater on the other, caused by spark erosion as the points open. When this happens, a feeler gauge no longer gives an accurate measurement of the gap.
A dwell meter measures the angle of rotation of the cam through which the points are opened and closed, and registers the dwell angle - the period when they are closed.
It can therefore be used to check the gap on worn points with more accuracy than a feeler gauge, and can be used without removing the distributor cap and with the engine running. The meter can also, of course, be used when fitting new points.
A dwell meter is usually part of an instrument used for various mechanical checks. Such instruments may be sold as analysers or test meters, and have to be switched to dwell for a reading.
How a dwell meter works
When the distributor shaft is rotating, the contact-breaker points open as the heel of the moving point is pushed outwards by a lobe of the cam, and close while it is over the flat area between two lobes.
If, for example, the angle of rotation between the centres 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 would be a typical dwell angle for a four-cylinder engine.
A dwell meter connected between the distributor or ignition coil and earth registers the dwell angle on a scale, and must remain steady at the prescribed figure while the engine is running. If the dwell angle is not the same for all cylinders, the result is rough running and poor fuel economy because the moment the spark-plug fires varies from cylinder to cylinder.
The dwell angle varies according to the make of car; check it in a service manual. There are two scales on the meter, one for four-cylinder engines and one for six-cylinder engines. Eight-cylinder vehicles are taken from the four-cylinder scale and halved.
Connecting the meter
Follow the maker's instructions. Normally one lead is connected to the side terminal of the distributor if it is outside the distributor body, or the CB (contact-breaker) terminal on the coil, and the other to earth.
Most modern cars have negative earth: connect the black (negative) lead to earth and the red (positive) lead to the distributor or coil.
On a vehicle with positive earth, connect the red (positive) lead of the meter to earth and the black (negative) lead to the distributor or coil.
Measuring the dwell angle
If you have just fitted new contact-breaker points, set them to approximately the right gap with a feeler gauge .
With the dwell meter connected, start the engine and let it settle to a smooth tickover. If it will not tick over steadily, check the cause(s) and rectify any faults .
The meter reading should stay steady at a steady engine speed. If it does not, increase the idling speed slightly until it does (See Preparing for carburettor adjustment).
Compare the steady reading on the appropriate scale with the desired dwell angle for your make of car.
If it is too high, the contact-breaker gap is too small. If it is too low, the gap is too wide.
Before you switch off the engine to adjust the gap, make the following checks. First ask a helper to press the accelerator to increase the engine speed slowly to about 1,000 rpm, then let it drop back to idling speed while you note the reading.
The angle should remain about the same, with no more than two or three degrees deviation.
Secondly, increase the engine speed quickly to about 1,500 rpm, then let it drop back to idling speed while you again note the reading.
The angle should again remain the same, fluctuating by no more than two or three degrees.
Carry out these two tests several times so that you can take an average of any differences in the readings.
If the reading constantly fluctuates more than two or three degrees, the distributor-shah bearing or advance-retard plate may be worn, or the cam itself damaged. Fit a new distributor (See Removing and refitting the distributor).
Switch off the engine before making any adjustments necessary to the contact-breaker gap.
Adjusting the gap
With the meter still connected, remove the distributor cap.
Using a spanner on the crankshaft pulley, turn the engine by hand in its normal direction of rotation until the contact-breaker points are held fully open by one of the cam lobes.
Loosen the contact-breaker fixings slightly (See Fitting and adjusting contact-breaker points), on some distributors you may have to remove the rotor arm to do this, and adjust the gap as needed. Reduce it to increase the angle if the reading was too low; increase it to lower the angle if the reading was too high.
Replace the rotor arm if removed, and the distributor cap. Switch on the engine and check the dwell-meter reading again. If the figure is still not correct, switch off and reset the gap.
Continue the procedure until you obtain the correct reading.
If you cannot get it right, make sure the right type of contact breaker is fitted. If it is correct, the fault may be a worn heel or pivot, or a weak tension spring. Fit new points (See Fitting and adjusting contact-breaker points). Check also to see that there is not excessive wear in the distributor-shaft bearing, which would cause the cam to run out of true.