What you need to do is confirm that with a suitable vacuum the plunger on the 3/4 point unit will withdraw fully and stay there. This is the “throttle plate actuator leakage test” in section A.3 of the manual. If it won’t then you have an air leak in your 3/4 point unit and will need to try to source a working secondhand spare or fork out £400 for a new one.
Assuming the 3/4 point unit is good then either you’re not getting a good enough vacuum due to a leak somewhere else or, if your vacuum tubes are definitely connected up correctly, the idle over run valve may not be closing properly when it loses voltage.
The idle at normal operating temperature depends on air at atmospheric pressure being able to get from the red hose connected to the top of the carb, through the blue one, through the overrun valve, and into the yellow vacuum hose. The valve closes when it loses voltage and this enables the manifold vacuum on the pink hose fully to withdraw the throttle actuator; like your test blocking the yellow port while pulling vacuum on the pink one.
Remember I said you’d need to understand the thing before you’d be able to get it working?!
