on older cars, there are two ignition advance systems in place.
to compensate for the shortened time where the piston moves, a centrifugal advancer is used to optimize the power stroke pressures.
to compensate for an increased volume of fuel (increased engine load), a vacuum advancer is used to give ample time to completely burn the increased amount of fuel.
if the vacuum advancer is not working, it takes a delay until the engine speed catches up and uses the centrifugal advancer but the total advance (centrifugal advance for increased speed plus the vacuum advance for the wider opened throttle plate) will not be reached. fuel consumption will increase and the power output is not at optimum