Logic tells me that the SU needs an extra shot of gas for the startup process, since there is no accelerator pump or dash pot to provide the increased fuel volume needed. Lowering the fuel needle provides the richer mixture and opens the venturi butterflies for the extra air needed for initial combustion (fast idle). Once combustion begins, the SU's operating vacuum is established and the choke can be turned off.

Even a fuel injected Volvo uses the cold start valve to achieve the same results all year round. A fouled cold start valve can prevent the engine from starting, causing the primary injectors to flood the engine, even in the summer. A faulty fuel pressure regulator can cause a similar malfunction and flood the engine. It's just that we don't have to do anything with a fuel injected engine, but it's still a similar mechanism.
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Cam a.k.a. CVOLVO.COM