Some constant-speed engines, like for power generators, water pumps, etc., had only "centrigugal" advance, controlled only by engine speed. Vacuum advance is a fuel-economy thing, allowing a few degrees more advance at light load, part throttle. The advance goes away quickly when the throttle is opened to climb a hill, pass, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!