Hi Malcolm
Rule number one: it is going to start. Remember that!
DON'T yet disassemble the dynastart to check the woodruff key - far too much like hard work, if the key turned out to be fine.
There's a much simpler way to check, using actions that you need to do anyway: if the key has sheared, the timing will be way out.
(And if the points plate has slipped, the timing will be slightly out.)
So, with pages 18 and 19 of your Mark D Instruction Book to hand (and apologies here for the granny-sucking, but novices may be reading):
1. Remove points cover and cylinder head.
2. Set points gap (this must be done first).
3. Now, get a decent indication of when the points open (you can't do this very accurately by eye):
Either: use a test meter, with one lead clipped to earth, and the other to the spring supporting the "moving" point.
The test meter is set on "Ohms" and needs the ignition OFF. It will show a reading when the points are closed.
Or: use a little 12V bulb in a holder with short lengths of wire. Clip the two wires as above.
The light needs the ignition ON (briefly, while you do the test). The light will light when the points are open.
4. Moving the kickstart lever by hand, get the piston to where it's coming up in the cylinder. As it gets near the top, start watching your test device. Stop where the meter JUST ceases to show a reading (or the light JUST starts to come on). This is the position at which the points open (and spark occurs).
5. (Turn off the ignition if using light and then:) Measure the distance between the top of the piston and the top of the cylinder. This should be 7/32" (or 5.5 mm).
If this is different, adjust the timing by slackening and moving the plate which supports the points. It's a bit "trial and error", as you have to guess the amount of movement, then tighten everything back up, before testing again.
If the timing was so far out that moving the points-plate can't make it correct, then the woodruff key has sheared, and the dynastart rotor has slipped round the crankshaft. If the timing does come back in range, then fine.
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Some final thoughts: what happened when you tried to start (with the kickstart)? If you got backfiring, then the spark is working, but at perhaps the wrong time. (Using the dynastart can disguise that.)
Try it with the points cover off (I've had mine short out the ignition!).
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Hope this helps!