I've had the throttle cable set up like this for a while:
As a few folk on the AOMCC forum have pointed out, it doesn't work very well. First off the cable outer doesn't have a good abutment in the stainless steel bracket; secondly, the volute spring (granted it is the wrong one) has way too much internal friction to return the throttle successfully; thirdly, the cable run is poor, both due to the bracket and the volute spring.
But mainly, with the Solex carburettor and its butterfly throttle valve, the cable return spring is the only method of closing the throttle - so if it is not working well, you are going into the back of the braking car in front (and I have done that before) or that expensive rebuilt engine is going to disappear in a mass of tangled aluminium.
So, now I have this:
I have removed the volute spring entirely, and I have a stiff coil spring in its place. It's attached to the dynamo strap with a stiff wire link, and it has a little brass sheet link to attached it to the carburettor cable clevis pin. The throttle returns beautifully now.
I need to adjust the cable path by bending that bracket a little more, and I'll plate that spring later.