Even using the best parts from a Motorcraft tune-up kit, old Autolite distributors don't hold up well in regular operation. As point-rubbing blocks wear down, dwell changes and performance suffers from changing point gaps and burned contacts. You would be surprised how many enthusiasts drive Mustangs with inoperative vacuum-advance units. Vacuum advance gets a car started under acceleration by retarding the spark at low rpm and handing off to the distributor's mechanical advance (advancing the spark) as rpm increases. Without vacuum function, engines don't perform well when it's time to accelerate. During cruise conditions, the vacuum advance maximizes fuel economy. This is the original shim-style Ford vacuum advance common in the '60s.