This is the Cardone distributor they are talking about:
So far as I know - the only complaints I've heard have revolved around excessive endplay. The excess travel will show up as a deviation in ignition timing as the gear moves up and down on it's helix. You may have to knock the pin out of the drive pinion and add a shim to set the endplay where you want it - and shims for the purpose are available at NAPA and others. Some of these distributors have come with adjustable vacuum pots - and some not. The adjustment is an "Allen" (socket head) set screw accessed down inside the vacuum port. Try a 3/32 hex for starters - some have been 1/8. Go easy on it - easily stripped. The effect of the screw is "counter intuitive" - turning it clockwise has the opposite effect you expect - it increases vacuum advance rather than restricting it. The Cardone unit will not come with a cap and it takes the later model cap - and the earlier '56 cap won't fit.
This is a map of the Ford advance scheme used for engines in the late sixties to mid-seventies - shown in distributor degrees. It isn't designed for maximum performance - more for warm-up, safe for warranty purposes and best gas mileage. Using a kit of springs from Mr. Gasket, you can boost the "bottom end' torque of your engine by increasing the rate of centrifugal advance - putting more in at lower engine speeds. This diagram below shows a modified advance curve that works with stock cams....adds centrifugal and reduces the top vacuum advance setting.
As you can see - it substitutes mechanical advance for vacuum at lower rpm ranges - allowing for engine acceleration (low vacuum) without losing the spark advance. Many try to simply increase the static timing (going from say 6º to 12º and more) without paying attention to the total - and the vacuum pot can easily add too much when she is up to cruise speed with low throttle (engine will go way too lean).
Steve Metzger Tucson, Arizona