Here’s the weasel himself, albeit in virtual form…. How can he be lost….. Perhaps he will turn up?!

Didn't think about pot holes to be honest. 

Hello         It's not those splines that were wrong  It was the ones in the Sun Gear that drives the Planet gears that was bigger Roger

That sounds about right. In my case I wanted to stop the Thyristor ticking over so that it was OFF. However if you keep the power applied to the Anode (the front hot end)  and cut the earth you will stop normal power operating whatever device you have but the Thyristor is still lurking. So when you remake the earth (but you do not necessarily want the Thyristor ON) the Thyristor does in fact power up and turn ON fully. By having the pull up resistor on the Earthy kathode  when the Thyristor is ON everything works as it should. The pull up resistor will pass 12mA but has no effect. However with the Earthy end open circuit the pull up resistor puts 12V on ther kathode so both sides of the Thyristor now 12V and stops current flowing - even the stuff that is lurking. To answer RedRoosters point - yes  a Flip Flop (Invented by the Frenchman Philip Philop) will do that basic function. My circuit has my version of a FF  but it is fashioned as a time delay switch.   The Thyristor and MosFet are required to shift the load in my case 400mA but it could be as high as 35A. I did use relays for this part of the circuit but they were not shake proof and could easily change state over a pothole.   Roger      

Oh, that way madness lies; let me shun that!

Is this a pulldown resistor? In the latest Binky video (where they are so far down the electronics rabbit hole they might very well be in the earths core), they talk about pull up/pull down resistors, which are used to ensure that the voltage on an open switch reverts to what you want.  These are used extensively in the speeduino (and I'm sure all other ECUs and electronic devices) to ensure that logic states that are wanted are achieved i.e. the thing you want sits at the voltage which relates to a 1 or a 0.  My understanding is that a pulldown resistor basically connects the switched outlet to ground.  So when the switch is closed, a pulldown resistor operates as a resistor and limits current flow allowing the logic output to remain at the hi-voltage state, but when the switch opens it allows current to flow between the logic output and ground to dissipate the voltage.  The analogy in the binky video is that they operate like a return spring on a throttle returning your logic signal to the required state (pullup being to hi, and pulldown being to ground). This is a level of electronic circuit design knowledge above my previous understanding, and sort of makes sense.  Turns out learning about electronic circuits and logic control is more interesting when the end result is horsepower! Who knew   Might be interesting reading if you're that way inclined! https://en.wikipedia.org/wiki/Pull-up_resistor

Oh no. What have I started.   Roger

If I had 1 pair running lean I'd look at the carbs, but both failing with similar faults at the same time. I fully rebuilt the carbs just before the engine build, bodies faced and bushed for new spindles. Jets converted to old style (no wax stats)  I'll check them over before I spend a fortune of an electric fuel pump setup, but I just don't see it being the likely cause.

Have some wild speculation, based on my personal carburettor woes.  Information very much worth what you paid for it  Could it be the plastic hoses to the jets?  My old ones went brittle and kinked causing fuel starvation.  Difficult see under the sleeves, but obvious once you move them out of the way. One other option is something wearing and causing a vacuum leak.  Throttle spindle being a likely culprit.  I understand o rings and/or vaseline can temporarily improve these.

Excellent photo, thank you.  Does that mean you don't have a coolant bypass unless the heater is on?