What fix did the British find for Spitfires' engine cutouts in a dive?

When performing a negative-g maneuver, diving hard, fuel was pushed toward the top of the float chamber instead of flowing down to the engine, causing a loss of power. Fuel flooded the carburetor with an overly rich mixture, and the rich-mixture cutout shut the engine down completely. German pilots could exploit this by diving steeply while opening the throttle, a maneuver the British aircraft chasing them could not copy.

Beatrice Shilling, an engineer at the Royal Aircraft Establishment at Farnborough airfield, invented a device that could be fitted without taking the aircraft out of service. She designed a thimble-shaped brass flow restrictor, its dimensions precisely calculated to let through just enough fuel for maximum engine power. The design of that restrictor was later refined into a simple flat brass washer.

Without fully solving the problem, the restrictor, together with modifications to the needle valve, allowed pilots to perform quick negative-g maneuvers without losing power. That improvement corrected the disadvantage of RAF Rolls-Royce Merlin-engined fighters against the German Messerschmitt Bf 109E. In early 1941, Shilling toured one RAF base after another with a small team, fitting the restrictors, prioritizing front-line units, and by March 1941 the device was installed throughout RAF Fighter Command.