Why does the choke on my 1985 Rolls-Royce Silver Spirit stick?

Question

Accepted Answer

Carburetted V8 engines in Rolls Royces of this era use a fairly simple choke system, based on a bi-metallic strip. When this strip is cold, it rotates the choke to the on position, thereby giving the engine a richer mixture (more fuel, less air). As the engine warms up, hot air plumbed from the exhaust system heats the bi-metallic strip, causing the choke flaps to revolve into their off position. There’s also a mechanical cam system operated via a linkage from the throttle pedal that sets the chokes on the first start-up and then maintains a fast idle once the chokes have begun to wind back into their off position.

The pipes that carry the hot air to the bi-metallic strip can crack or fracture over time (and heat cycles) and this can mean the strip won’t get hot enough to turn the chokes off. Sometimes the mechanical cam system can become sticky, causing the fast-idle condition to persist. Next time you notice the engine fast idling, try giving the throttle pedal a short, sharp stab to see if that brings the idle back to normal. If not, you may have to replace the hot-air pipes. Removing the choke flaps from the rest of the carburettor will also take the choke out of the equation, but you might find the car is difficult to start and won’t run smoothly in winter.