I have the Yellow & White wire connecting (under the main frame, above the carburetors) to the Boyer unit white wire AND to the relay terminal C-4.
This is wrong. The White wire into the Boyer
-Bransden "Transistor Box" should be connected only to the White/Yellow wire from the handlebar kill switch.
Starter relay C4 is (should be) only 'live' when the starter button on the handlebar is pressed, this is (should be) the only thing that operates the relay. I'm assuming you aren't intending to ride with the starter button pressed constantly? If so, connecting relay C4 to the B-B Transistor Box White wire does not serve any useful purpose.
7) The T160 machine has ... a ballast resistor fitted in the circuit.
This must be removed and the white ignition box wire connected to the live wire feeding it with the ignition on.
... the "live wire feeding it [the ballast resistor] with the ignition on" is the White/Yellow wire from the handlebar kill switch.
In the new wire between that T junction and the relay I have a 7.5Amp (US rated) fuse.
The wire from relay terminal C4 was/is [b]only
[/b] to bypass the ballast resistor when the electric starter is in use.
As the ballast resistor is removed when any electronic ignition is fitted, relay terminal C4 and any wire from it to the ignition become redundant.
Therefore, there should not be any "T junction" - the White wire into the B-B Transistor Box should simply and only make an end-on junction with the White/Yellow wire from the handlebar kill switch.
A 7.5A fuse only for the electronic ignition can be fittd anywhere between the kill switch and - in your case - the B-B Transistor Box; a convenient place
to fit the fuse is in the end-on junction between the Transistor Box White wire and the kill switch White/Yellow wire.
Fascinating little buggers.
Is your T160 a bike to ride or just an exercise in seeing how complicated you can make the electrics?
I should splice in a diode in the Yellow and White wire
A diode between the kill switch and the B-B Transistor Box cannot and does not serve any useful purpose on a bike to ride.