Morris Minor Owners Club

Exactly......feeler gauge (or to be honest a 'good eye) is fine.........

Exactly, either way is fine

An interesting discussion, now did the original poster get his problem resolved? Could we have an update, please?

Surely the gap setting is primarily for when you don't have access to a dwell meter. It gets it into a ballpark area. If you have access to a dwell meter then use that! I agree the gap at 60 deg dwell is the correct gap, it doesn't matter what it measures.

Very few have 'dwell' meter - it's an unnecessary complication (like colo(u)r tune and vacuum gauge) . The gap is easily set in seconds by eye - or with a feeler if you don't know what 15 thou looks like........ Fiddling about with the gap won't make the slightest difference to any standard A series engine...... As long as there IS a gap - it will be fine. I'm still 'dying' to know how the dwell meter works...

I have measured the Dwell Angle by temporarily fitting an ordinary circular angle meter/ruler from primary school times on top of the distributors' rotary arm.
Turning the engine over I mark the point when the points close [ indicated by a lamp p.e.]
It is this time the coil begins to be charged with current.
When the points reopen , again this I mark on the ruler.
At this time the coils current flow is interrupted giving a huge magnetic change inside the coil und thus miraculously producing the spark at its output [or so].
The difference between those 2 markings should be 60° as stated in the manual.
Turning the dizzy further, when the points close again the difference between that and the previous mark should be 30° giving 60°+30°=90° which is always ¼ of the dizzies' complete revolution of 360°.
I can only alter the dwell angle by altering the gap of the points.
At the end it turned out that the gap was .4mm measured with a feeler gauge.
A neighbour of mine once tried to measure the dwell angle with his advanced electronic measuring device which did not cope well with the Morris Minor 1000 giving quite a wrong result.
I might add that you can measure the dwell angle at any time but not the gap, once the points have run in and formed pits.

So - 16 thou!! Hah. Well done doing the test. It's the condenser that charges up - not the coil......there is no 'magnet'.

'60 degree' equates to 15 thou

On a 23D/25D/43D/45D 4-cyl Lucas distributor it does, but it is not a unique relationship. Other distributor designs will be different, as was the early distributor fitted to the MM. The relationship between dwell ° or % and point gap is a function of the number of cylinders and the distributor cam lobe design - the actual value is then determined by the ignition system design requirements.

Following the various comments about what makes the ignition system work, I have tried to out together a complete description using a combination of sources and previous knowledge - comments please!

The ‘modern’ coil ignition system, comprising coil, points, condenser and distributor, was developed by Charles Kettering, of what became Delco, in the early 1900’s, and it became universally adopted by the automotive industry.

The coil has two windings, primary and secondary. The primary winding is wound with a small number of turns and has a low resistance, so applying the 12v battery voltage to this winding causes a sizable DC current to flow. The secondary winding has a much larger number of turns, therefore acting as a ‘step-up transformer’, and is designed to produce a large voltage spike through ‘induction’, when the current in the primary coil is suddenly interrupted.

The induced secondary voltage is proportional to the rate of change of the magnetic field through it, thus opening a switch quickly in the primary circuit (the points) to drop the current flowing to zero, will generate a large voltage in the secondary coil according to Faraday's Law. The change in the early Minors to ‘high lift points’ was to open the points quicker, and produce a higher secondary voltage, thus ‘fatter’ spark.

The current does not flow instantly because of the inductance of the coil. Current flowing in the coil builds a magnetic field in the core and in the air surrounding the core, in effect 'charging' the coil, and the current must flow long enough to store enough energy in the field to be able to create a spark across the sparkplug gap. Once the magnetic field has built up to the maximum level determined by how long the points are closed for (the 'dwell' time), the contact breaker opens and the high secondary voltage (20kV – 40kV) causes a spark across the gap of the sparkplug to ignite the fuel/air mixture.

The downside of the design is that the sudden interruption of current in the primary winding generates an inductive back-voltage in the winding, which, in early systems, tended to cause sparking across the points. The design was improved by placing a condenser or capacitor across the points, so that the voltage surge was absorbed by charging the capacitor, rather than cause destructive sparking across the points.

The capacitor has two functions:

1. It absorbs the back EMF from the magnetic field in the primary winding to minimise contact points burning and maximise the points' life

2. The primary winding and the capacitor form a tuned circuit, and as the stored energy oscillates between the inductor formed by the coil and the capacitor, the changing magnetic field in the core of the coil induces a much larger voltage in the secondary winding of the coil – not sure if this was part of the original design thinking or an ‘accidental’ benefit?

Modern systems, and aftermarket ones such as the Aldon Ignitor, use a ‘fast opening’ transistor switch instead of the points to interrupt the primary current. The benefits are constant 'dwell' (in the case of the new Accuspark system, programmable dwell), and claimed higher secondary voltage through faster switching and collapse of the primary circuit current.

I know this is an old post, but I just happened upon it. For what it is worth:

Richard is 100% correct. The contact closes, and connects the coil directly to the battery, a current builds up in the primary (low voltage) side of the coil. The time it takes to build up is determined by the coil inductance and the system resistance, it is not instantaneous. The current flow creates a magnetic field that stores energy. You need this energy to create the spark in the plugs. More energy more spark.

But the maximum current in the coil is determined by the systems resistance and the battery voltage. No point (pardon le Pun) in keeping it on too long. This just creates unnecessary heat in the coil.

Inductors don't like a change in current. If you interrupt this nice current that is now flowing, the inductor tries to force the current to continue flowing, but where to, there now seems to be no circuit??!!

Aha says the inductor. If I increase the voltage at my terminals to a very high value I will eventually get some current to go somewhere!

So, to compensate for the sudden interruption of the current, the coil voltage increases on both low and high voltage sides until you get an arc (spark) and the energy is dumped into the spark plug.

The capacitor also acts as a circuit for the current, and as Richard has said has 2 purposes, the great thing about the 'oscillation' is it also increases the duration of the spark.

I have a dwell meter and as Roy says there is a direct correlation between dwell and gap. But only if the distributor cam is not unevenly worn. The dwell meter is there to dynamically check % time the coil is 'on' regardless of the wear on the cam. I have never opened mine up, but the basic circuit for a "mark-space ratio" circuit is a resistance-capacitor circuit connected to a volt meter. Properly calibrated it can give an accurate measure of the ratio between coil on time (dwell) and off time, which can be presented as degrees.

As pointed out, mapping is the purist way of optimising dwell throughout the engine speed range.

Well explained chaps, as a retired marine engineer even I could understand the terminology and reasoning behind this article