In 1926, John Kenworthy developed a further singleseat fighter based on the F.4 Buzzard, but utilising a 330hp A.D.C. Nimbus six-in-line water-cooled engine. Usually referred to as the "Nimbus-Martinsyde", it utilised an essentially similar airframe to that of the Buzzard, but featured modified vertical tail surfaces with a horn-balanced rudder and revised aft upper fuselage decking. Its intended armament comprised two synchronised 7.7mm Vickers machine guns, but these were never installed. Two prototypes were completed and flown, both participating in the King's Cup Air Race of 9-10 July 1926, and, in 1927, the first prototype was modified with faired undercarriage legs and cylinder head fairings. No orders were placed for the Nimbus-powered aircraft and both prototypes were scrapped.