Carburetion
SU Carburettor Needles
While Rolls-Royce used its own in-house carburettor designs, SU (Skinner’s Union) units began appearing as standard equipment or popular period upgrades as the company sought better performance and reliability in the 1930s.
20/25 hp (later series): While early 20/25s used the Rolls-Royce "two-jet" carburettor, many later series (starting around 1934–1935) were fitted with SU carburettor to improve breathing and cold starting.
25/30 (1936–1938): This model replaced the Rolls-Royce carburettor with a Stromberg downdraught unit (specifically the AAV type) with an accelerator pump. It was a major shift for the company toward using proprietary American-designed components for better cold-start performance.
Wraith (1938–1939): Continued using the Stromberg downdraught setup, often paired with a water-heated inlet manifold to prevent icing and improve vaporization.
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