still not starting

[Barnsey]

OK, taking onboard the resistance in the meter,  the battery is showing 6.4V but I am seeing 5.4V at the coil, so I am losing a volt in the wiring.  I replaced the lighting switch during the project, but not the ignition switch. Hence I have taken a leap of faith and ordered a new ignition switch at around £50 (ouch!)  and we will see what effect that has. 

[Derek996]

A 1V drop is still a fair bit (20%), You should make serious look to see where the resistances are that are dropping the voltage; the switch and every connection from the battery to the coil as well as the earths.

[Barnsey]

Yeah!  My gut feeling is that this is a host of small issues, rather than one big issue, and I will eventually knock them all down, but it does make me feel like I am chasing shadows. You know what I mean? The compression is just slightly low. Resistance here is just slightly high. Voltage there is just slightly low, maybe the fuel is just slightly lean, etc.   

[Cyclops]

Always try the easy (obvious) things first. Change the spark plug, they can fail to spark under compression. Members here have covered all the bases, wiring connections, H/T leads etc. Worth checking the points gap, for the sake of 5 mins.

[rhyatt]

If you start with 6.4 v with ignition off and then turn the ignition on , if the points happen to be open , then there'll be no change in battery voltage.

 the volts will certainly drop if the points happen to be closed (that's why the ammeter kicks over), as the coil will consume around 3 or 4 amps.

Normally, the voltage is still fine to start the bike and then the engine recharges the battery.

[rhyatt]

I've sent you meter readings in the past to check the quality of the points , from the +ve side of the coil wire down to earth via the points closed. Should really be 0 ohms proving the points are good.

But you say old and new coils give a good crack spark with your motobatt battery , you're fine there then.
Hopefully, this healthy spark is in the right place timing wise and I've gone through with you how to check that.

I sent you valve adjustment instructions , did you check the clearances and adjusted.

Yes , from memory , your comp ratio was a bit low , but ok to start (and could well improve with running
There's not much left

[Steve.S]

It's starting (no pun intended) to look like there is a problem with your wiring loom or connections.
So bypass the loom, and remove all the coil wires. Connect a new wire from the coil to the points, and connect a wire from the coil to the battery negative and start your engine. It shouldn't take too long.

[iansoady]

I have a few cables with crocodile clips on both ends to check this sort of thing. It just takes a moment to bypass suspect components.

[V500]

I seem to recall the by-passing of the wiring has been suggested before in this thread, but even still, the loss of 1V seems to be a red herring.
They were never that finicky when plodding to work in all weathers on minimal maintenance.

[iansoady]

It is possible to see a good voltage somewhere but as soon as a load is applied it disappears. Often happens if LEDs etc are in the circuit.

[Derek996]

...Often happens if LEDs etc are in the circuit.

Please explain?

[iansoady]

You can get a back feed of low current through them. I once spent ages trying to isolate a fault in a caravan - a wire had come off a switch but an LED indicator was letting voltage through so it looked as though it was getting where it was needed.

[neil1964]

I missed the result of temporary removing all wires from coil etc.
Just a wire from pos on battery to pos on coil.
-ve battery to ground
-ve coil to distributor.
With clean points, new coil and plug, tickled carb, half choke you will get something.
Have you done EXACTLY as above?

[rhyatt]

Just to clarify , if you're original pos +ve earth , then the + of the coil must go to the points and the battery neg -ve on the battery to the ignition switch or straight to the neg of coil
This'll give you a spark nicely when you kick it and the points open and close

[Derek996]

You can get a back feed of low current through them. I once spent ages trying to isolate a fault in a caravan - a wire had come off a switch but an LED indicator was letting voltage through so it looked as though it was getting where it was needed.

I see what you are getting at. The cathode leg of the LED will show the supply voltage when there is no current flowing through it. Although I'd expect that the LED would light up when a load is applied. Any circuit with a high resistance in it can show a full voltage until a load is applied.