OK, progress!
I tried three basic volt drop tests. The earth (negative) side to the battery to the chassis, effectively bypassing the battery isolator, is losing 0.23 Volts while cranking. The main power cable and wiring to the coil is losing 0.95 Volts while cranking. The Voltage at the coil is 12.5 Volts and during cranking is 9.45 Volts, which is a difference of just over 3 Volts loss. 0.23 + 0.95 = 1.18; 3.05 - 1.18 leaves a cranking Volt drop in the battery of 1.87 Volts.
I don't feel the volt drops during cranking are worth worrying about.
We have just came back from a blast through the lanes up and down our biggest hills locally. At first I couldn't get the car hot enough to refuse to start but it is blowing hard and quite cool here so I suppose it's no great surprise. However, I noticed the engine temp rising a little so I pulled up at a small refuge and tried to re-start the engine... No dice! So I quickly installed one of my ballast resistor coil. Instant re-start, switched back to the 'original' 12V coil and no-go.
Cracked it, we thought. I drove homewards about 4 miles on an 'A' road, on the last down hill I stopped and tried to re-start on the Ballast resistor coil. No-go, we re-connected the original coil and it started perfectly.
It seems the coils are getting too hot.
Whether this is because of the ambient temperature under the bonnet or some fault in the distributer I don't know yet. The dwell is set as it should be, the points are new. It might benefit from a new set of plugs but given it runs so well I don't think that is the issue.
Ambient may be a factor but I don't know yet. My next move is to re-locate the coil to in front of the radiator. That is my coolest place, although not the driest, either that or make a hole through the bulkhead for the coil lead and have the coil behind the dash. That will be another day now, I have much else to do but at least I have a much better idea of what's wrong.
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