Wow! Sounds like the exact same issue I am having.
Mine is running a standard 1600 with 2L weber carby. Carby was thorougly cleaned with carby cleaner a few months before symptoms started.
I had the battery in the boot and worked fine for years. As part of my trouble shooting I moved it back to the engine bay.
I have new (all less than 1 year old): battery, coil, points, condensor, dizzy cap, plugs, leads, start motor (new not rebuild).
The symptoms are:
- starer motor engaging but not turning the motor with enough force
- or starter motor not quite engaging and getting the whirring sound (pinion free spinning or slightly engaging)
- intermittent voltage drops while cranking (may be causing the above)
I left the car off the road for a few weeks until I got a chance to trouble shoot it. The night before I charged up the battery. Come morning I tried to start it with various throttle positions. Gave up and added some WD40 down the carby throat and it started to kick. Finally got it started.
It has now been starting every morning and evening with a little persuation. It dies the first time it fires up about 50% of the time, then fires up fine the next go. Left over the weekend it sometimes needs some WD40 to get going but then it is good for the week.
If I stop the car and try and start it 15-20 minutes later it wont start. An hour or so and it is good.
The voltage dropping makes me think it is electrical so I want to try replacing the solonoid and/or ignition switch but haven't got around to it. The solonoid is separate to the starter motor on the 1600.
Other ideas I have are possible compression loss in 1 or more cylinders. I have tested with one of those press-to-seal type gauges but I don't think I was getting a good enough seal. I have borrow a screw in gauge and have yet to try it out.
The car runs and idles fine once it is warm with no smoke when starting or accelerating.
I will report back with any more test results as I get around to it.
Good luck.