| The reason for a second cable to shut the throttles has to do with earlier carbs and especially carbs using slides as opposed to butterflys. Slides can easily stick open under deceleration when the engine is pulling a strong vacuum. The slides are being sucked into the engine which makes them stick. If you'd fit a return spring big enough to overcome this effect, you'd not like the stiffness of the throttle for opening.
Carbs that have butterfly throttles are far less prone to this problem and as you say the later fuel injected models which utilise butterflies only use an opening cable.
Yes, you can retrofit them to older carbs, but you have to make a judgement on the throttles potentially sticking open. | 
Hybrid Mode
