The 1 1/2" HD4 SU Carb6th of November, 2019
The HD4 is the smallest of the HD series carb, which can be identified by the float bowl having 4 small cheesehead screws attaching it to the main body. They were fitted to more premium classic cars, they had cork seals (later rubber ones) on the throttle spindle to reduce airleaks and give a smoother idle.
Identification: The HD4 has a 1 1/2" bore, front flange with two horizontal threaded holes the same as the HS carbs. They always have the full square mounting flange with 4 bolt holes. The jet mechanism is completely enclosed inside the carburetor body.
Most HD carb setups have a solenoid operated AEC (Auxiliary Enrichment Carburettor) which is effectively a very small carburetor that provides a very rich mixture. See here for more information.
Unlike HS style carbs, the HD carbs can have the float bowl mounted on either side of the main body.
Photos of HD4 SU Carb
If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: firstname.lastname@example.org