Coolant Temperature Override (CTO): Dual-Function
This valve supplies manifold vacuum to vacuum advance at both cold and hot coolant temperatures. At normal coolant temperatures, carburetor ported vacuum is supplied. When coolant temperature is below 149°F, manifold vacuum enters through port "3" and is applied through port "2". This allows full manifold vacuum to distributor.
From 149-220°F, port "3" is blocked and ported vacuum enters through port "1" and connects to port "2". This allows full ported vacuum to distributor. Above 220°F, port "1" is again blocked and full manifold is applied to distributor through port "2". See Fig 2.