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Prior to 1968, Autolite distributors had a single vacuum-advance port that did one thing--advance the spark. Beginning in 1968, Ford went to a dual advance/retard unit with two vacuum hoses. This happened because of the need for reduced emissions. Under acceleration, it advances the spark. During deceleration, it retards the spark to reduce emissions. Despite Ford's best efforts, the dual advance/retard system (called IMCO for improved combustion) never worked very well.