RR Merlin Booster capsule failure.

Belated thank you’s for the excellent replies . It was quite common on a heavily loaded bomber Mosquito at take off . Did the Lancaster suffer from this as well ?

The SU carburettors on most Merlins are a twin barrel type with the fuel metering needles controlled by aneroids, or capsules. The fuel mixture in one choke is controlled by an atmospheric pressure capsule, so that the fuel / air ratio can be maintained with changes in altitude, and the other is controlled by boost pressure, the “boost capsule”, which increases fuel with boost pressure.
The capsule can fail in two ways: If it is punctured, it will force the needle into its fully weak position, if the internal spring breaks or becomes weak, it will allow the needle to move away from the jet, causing a rich mixture.

I`m not aware of this problem on specific aircraft. It is known that it can happen, but it wasn`t one of the more common faults in service. A weak mixture in crusie under boost could tend to cause the engine to overheat, whereas too rich a mixture would reduce the range of the aircraft, along with a drop in power output accompanied by black smoke.

Pete

I think it is about +54lbs

What is 140″Hg in real money ?? Old duffers like me only remember lbs of boost.

Not professing to be a Merlin expert, but a damaged aneroid in the Boost Control may prevent full opening of the carburettor throttle valves, therefore reducing engine power output. The boost control is used on some supercharged engines (mainly high powered) to limit the maximum boost pressure produced as the altitude varies. At low altitudes, without the automatic boost control, it is possible to overboost the engine by opening the throttle valves too wide, running the risk of damaging the engine. The Reno Race engines have the boost controls bypassed or disconnected to allow boost pressures of up to 140″ Hg.