Help request with electrical problem
I have an intermittent problem with my ignition on my 1972 2.0l which is undergoing a complete restoration. Fuel injection has been replaced with twin carbs

Sometime it starts OK and then next time will fail to start on crank

Turn Ignition on so oil and ign light comes on OK and then when you crank it looses all power ie no starter crank and dials for volts, temp etc all loose complete power light as if something is grounding out the circuit

Fitted and new ignition switch and same issue. The car does start when hot wired using wires directly into black mail connection block without ignition switch

The head lamps, indicators, hazzard all etc work OK but horn , fog lamps and interior light doors switches do not ( although interior light does work OK and the door switch do appear to work properly)

So to be clear it is really a problem with cranking?

Disconnect he battery and then Make sure that all your connections on the starter are tight and correct, that’s where the car picks up power for the rest of the systems

Sounds like the wire to the coil, in the engine harness is bad. I'd suggest getting a new engine/ignition harness from Jeff Bowlsby. The original wiring in the engine bay gets brittle and cracks with age, causing bad connections and corrosion in the wire itself.

For starters, you can check out the continuity of the wire from the 12 pin plug to the coil, it's black with a red stripe

Well, sort of, but not exactly. The three main red power leads for the car connect to the battery, and the battery connects to the starter.

When the fuel injection is removed you lose a lot of grounds and adding an additional body to engine ground is needed. There is a hole on top of the fan shroud that works well for this.
Check all off your grounds and clean all over the car.
Also add a start relay at the starter to take the load off of the switch.
Hope this helps

Many thanks for comment
I have installed new gearbox earth strap and battery earth cable, would you still recommend adding another then to the engine? Doesn't the gearbox earth have the same function?
Starter motor, battery, coil, leads, distributor etc are all brand new. I did remove paint and clean all body earth points but will do so again to be sure

We cant seem to isolate why the fault is intermittent and has no issue cranking when hot wiring it. This would suggest it isnt and ground issue at the earth points but perhaps it is grounding out somewhere else on the loom perhaps???

yes add a second engine to body ground.
Had this very issue back in the 90's when i had a car converted to carbs. The added ground strap fixed it and then also adding the relay to save the wires and ignition switch.
These cars hate dirty grounds. somebody posted a diagram of all of the ground lug locations on the chassis and it would be a good idea to service every one of them or you may be chasing gremlins instead of enjoying the car.
Just my 2 cents

I seem to have the same issue, a year ago I went through SO MANY ground and contacts, cleaned and sanded to work for one year then come back as before.

That sounds like a bad ignition switch. I know you said you have a new one but no crank is typical of bad ignition switch.

It would t be unheard of for the cheap crap switches to be bad out of the box. I think 30 guys here could attest to that...