How silly of me. Either 1108 or 1301, lampredi engine. see photo. The S/W flap is inside the inlet section where the warm air duct joins the main inlet duct.
A small brass capsule is intended to expand and close off the hot air intake by the crude plastic linkage. I don't think these ever worked very well. Ten years ago I could only find one that worked, out of a collection of five.
What I would suggest is to remove the dysfunctional parts and fit a screw to the flap pivot, so you can set the flap position to 'winter' for the present conditions. You might also need to check/replace that relatively-expensive aluminium duct pipe from the exhaust tin shield. It is worthwhile in the winter - helps prevent carburettor icing and generally enhances the carburettor operation during the warm-up phase.
Aha, funny you should say that. I pulled one apart, and was thinking it must be something like this. Then I read your post. Is this an example of where Haynes is not up do date? I read about a manual adjuster, mechanism but that must be for another type, or earlier! But how much earlier can you get from 84.
don't break the flap or leave it open in the winter...my first fire engine would freeze the carb in winter and slowly come to a halt,,,after 20 mins .tiny engine heat would melt ice in carb and away we would go!!!took me a long while to suss that!