Cruise control adjustment

[bnoon]

I repaired some of my wiring harness over the weekend and functionally the cruise control is working. But say I set it at 70 MPH, it will slowly lose speed down to about 60, then run up to about 75, before again going down to about 60, repeat. How are these older units adjusted for speed control? This is the single button vacuum operated system with two speedometer cables. Vacuum lines and diaphragms seem to hold vacuum fine.

I guessed it would probably need adjustment since I pulled this transducer unit from a 1976 Malibu in the scrap yard since mine was stripped out due to an under lubricated upper speedometer cable. I also pulled the 28% increase speedometer cable adapter unit from this car as well. Cost was less than 20 bucks! I love pull your own auto parts!