Originally Posted by motolab
.BST40 carbs have cold start enrichment circuits instead of chokes. A choke is closed when an engine is cold and opened when it is warm, whereas a fuel enrichment circuit is opened when the engine is cold and closed when it is warm. A choke works by restricting the air flow, whereas an enrichment circuit works by adding extra fuel. A choke requires a fast idle cam or something akin to it (or you have to hold the throttle open manually), whereas an enrichment circuit adds a little extra air simultaneously with the extra fuel, so the fast idle is already built in.
I do believe Mill understands what the enricher circuit (choke) does. Read his post ... he said he "broke" the choke. If you had ever worked on a DR then you'd know how flaky the plastic enricher unit is. It uses a plastic fitting that threads into Carb. Easy to screw it up. This is why many swap from cable system to manual system.