Hey there dizzy experts- my recently acquired (mfr'd 5/77) is all stock and original, with a single diaphragm one hose vacuum line dizzy, semi-electronic w/ igniter. I don't know if it's a retard or advance unit. when I manually(mouth!) apply a vacuum, it does rotate the points plate as hoped. My thoughts are, (please correct me if wrong) under light throttle= high vacuum= plate turns= retards timing. Heavy throttle= low vacuum= no retard/more power. But... if it's an advance mech., then it doesn't make sense!?! Please enlighten me? Thanks to all!