So, having pretty much exhausted every avenue of investigation, I began to back track and consider even the most unlikely possibilities. One of these was the coil. A new replacement coil, yes, but what had I got to lose? I had kept the old coil and cleaned it up as much as possible, especially the contact inside the tower. It was rusted, and covered in a brown lumpy deposit. I connected it up and ran the water, turned the key and the engine spun much better and gave a few coughs and splutters. I don't have the "best" starting procedure for this engine, having read numerous and often conflicting approaches. Pump the gas 8 times, return the throttle to about one third forward, or pump twice and return throttle to middle position, or pump 5 times and push throttle fully forward etc. etc. Anyway, it was in the full forward position and I turned the key again and it fired up! Woo hoo! I pulled the throttle back immediately and she died. But, she had started!!
So, a new coil was ordered which arrived this evening. I put it on and the signs are very encouraging, but the battery was weakened and the weather was deteriorating likewise, so tomorrow a fully charged battery will be hooked up and fingers crossed. I feel like I'm out of the woods on this, and will be able to fine tune things once she's up and running. Many thanks for all the inputs and responses. It's always useful to have a different perspective and so avoid the tunnel vision we can sometimes develop when trying to diagnose an issue.
I still have to sort out the mystery of the missing wires. As mentioned, my coil connections have a lug for the + side with two wires attached, but on the - side there is only one grey wire attached. The remains of another grey wire can be seen inside the lug, which was obviously cut off at some point. Perhaps the grey is spliced in further along, This has to be checked out. But the existing grey wire, which should be my tacho isn't. There is a grey wire on the back of the tacho, but when I test this, it gives continuity with the grey wire on the sender on the right hand side of the thermostat housing, as you look at the front of the engine. On the opposite side of the thermostat is the other sensor (water temp) with one wire attached, Tan but missing a Blue wire as shown in any wiring diagrams I've studied. The temp gauge pegs full when this Tan wire is grounded, so the gauge checks out.
What's the sender on the right hand side for and why would the grey wire from it connect to the tacho? I'll be taking a good look behind the dash at the gauges and see if I can make sense of that particular bowl of spaghetti, but any thought most welcome. Thanks again!
