Throughout the twenties and thirties, Francis Barnett continued to make a wide range of two stroke machines including the Pullman and the Aza, and a whole series of models named after British bird species such as the Plover, Merlin, Kestrel and Lapwing. The War Office did investigate the suitability of Francis Barnett machines for use in the military, but there were concerns over the reliability of two stroke engines.

The 122cc Snipe was launched in 1939, though production was short-lived as Britain became embroiled in the Second World War. As was the case with many of the British motorcycle manufacturing companies, their factory was badly bombed during the conflict, and it is felt that this may have been partially why the contract for the supply of lightweight airborne models which was eventually placed with Royal Enfield and James.

After the close of the War, Francis Barnett recommenced production, focusing on their lightweight range of models as they provided an extremely cheap and economical mode of transport. It was advertised that the Merlin L.51, launched in 1946, was capable of going 250 miles on a single tank with a capacity of about two gallons.

In 1947, Francis Barnett was incorporated into the AMC (Associated Motor Cycles Ltd) group. For a while in the late 1950s, AMC’s own 249cc engine was used in the Cruiser model, but it was never found to be as reliable as the Villiers engines used previously. Despite this, the AMC engine gradually replaced Villiers across a whole range of models. Francis Barnett and James models were virtually identical, except for color and badges.

In 1962, production was moved to the James factory at Greet. But sadly in 1966, production of Francis Barnett motorcycles ceased, primarily due to the financial problems being experienced by James.