Your correspondent, Martin Johnson, is correct in his presumption that a fuel cut-off device would not work well were it to operate between the tank and the float chamber on an SU carburetter. However, that used on the Maestro operates immediately before the jet, so that plenty of fuel is always available, when needed.
In practice, the SU system incorporates several safeguards against the pitfalls mentioned by Mr. Johnson: it is used in conjunction with an automatic choke which is activated through electronic devices sensitive to the relevant factors, such as coolant temperature, engine compartment temperature, engine speed, throttle position and rate of change of engine speed. By this means, the throttle position is manipulated to maintain the engine idling speed at the pre-set level in all conditions, and the cold start device is manipulated to provide a suitably enriched mixture whilst the engine is warming up.
The fuel cut-off device operates only while the engine speed is above 1,200 r.p.m., only while the rate of change of engine speed indicates that the engine is being used to slow the car, and only while the engine compartment temperature is above freezing point. When the engine is disengaged from the road wheels, the engine speed falls much more rapidly than it does when on the over-run; this rapid change in engine speed is sensed and the cut-off device then does not operate.
On the open road, of course, there is virtually nothing to be gained from an over-run fuel cut-off device but, in town !raffle, Austin Rover suggest that an Improvement in consumption of some three per cent can be expected. Duns Tew, Oxford J. G. MILLWARD