My daughter had a Triumph Spitfire with one of those carbs on it, back when she was in high school (She's a grandma now). As I remember, it was a pretty good carb, although I had to rebuild it once. It's in the same category as the old S-U "constant depression" design, where a sliding piston tries to maintain the same manifold vacuum at all cruise throttle settings by sliding a piston up and down to change the size of the venturi, with a needle valve attached to it for mixture control. The difference is how the piston gets moved- - - -the carb in the ad, as well as the Harley Davidson carb I mentioned use a rubber diaphragm to move the piston. The old S-U just had a close-fitting piston in an aluminum cylinder- - - -no diaphragm. The Brit engine S-U carb mechanics interpreted "constant depression" as their state of mind when they tried to keep the things synchronized!
Jerry
