There was some interest in this article, so here is a summary of it.
Will
Summarized from _Your Classic_, May, 1992, p. 40-42.
A dwell meter is used to measure the distributor's dwell angle. The
dwell angle is the angle of rotation of the distributor shaft, and thus
of the contact breaker operating cam, during which the points are closed
and the coil is building up the energy required to produce the next
spark at the relevant plug as the contact-breaker points open again.
On a four cylinder engine the angle of rotation between the peaks of the
cam lobes is typically 90 degrees. The period during which the points
are closed something like 52 degress. A 6 cylinder engine- the peaks
of whose contact-breaker cam lobes will be at 60 degree intervals
(360/6) - might have a dwell angle of around 35 degrees.
Check your manual for the correct dwell angle reading either in per cent
or degrees.
The dwell meter is connected between the low-tension side of the
distributor and ground (or earth). It measures the angle of rotation
of the contact-breaker cam through which the points are both open and
closed, and registers the dwell angle - the period in which the points
are closed- on a graduated scale.
Since this is measured while the engine is running it is far more
accurate than using a feeler guage, especially if the points are worn.
Directions:
1) Hook up the dwell meter. On a negative ground car the positive
terminal goes on the distributor terminal.
2) If you have just installed new points set them roughly using a
feeler gauge. Put grease on the cam, etc.
3) Start the engine and let it get into a smooth idle. The dwell meter
should give a steady reading at idle. If it doesn't, increase idle
speed until it does.
4) If the reading is too high the gap is too small. If the reading is
too low then the gap is too wide.
5) Before readjusting the gap slowly increase the engine speed to
about 1000 rpm and then let it drop back to idle. The reading on the
meter should stay about the same with no more than 2 or 3 degrees
movement either way.
6) Next, increase the engine speed quickly to about 1500rpm then let it
drop back to idle again. Again, the reading should stay about the
same and not vary more than a few degrees. Do these last two steps
3 times. If the readings fluctuate a lot it is likely that the
distributor shaft bearings or the contact breaker plate are worn. Or
even that the cam itself is worn unevenly or damaged.
7) Switch off the engine and remove the distributor cap.
8) Turn the engine so that the operating heel of the contact breaker is
resting against the very peak of one of the cam lobes, either by putting
the car in gear and carefully rolling it forward, or, prefereably, by
placing the appropriate spanner over the crankshaft pulley bolt. TURN
ENGINE ONLY IN DIRECTION OF NORMAL ROTATION. (Don't roll the car unless
you have a flat floor either ;-)
9) Loosen the contact breaker fixing(s) very slightly and use a
screwdriver in the slot provided to carefully lever the fixed half of
the contact-breaker either further away or closer to the moving one as
required. A feeler gauge (usually .0015") can be quite helpful to avoid
overadjusting.
10) Tighten the contact breaker screws, refit the distributor cap, and
start the engine. Check the dwell meter reading again and repeat
as necessary until you get it close enough. If you can't get it to work
then the problem may be either the wrong kind of points, a worn contact
breaker operating heel or pivot, or a weak tension spring. Also check
that there is no sideways movement in the distributor shaft because of
worn bearings.
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