An Electro-Valve is an electrically operated valve.
On Fiats, it's used to control the supply of engine vacuum to operate one or more vacuum operated units - afaik this is to control emissions.
The 'button' you mention is a test button to operate the electro-valve and iirc is pressed and held while adjusting the fast-idle speed to 1600 rpm. While pressing the test button, you have to bring the engine speed up to 2500 rpm and allow it to drop down to the fast idle speed to see where it's at and you then adjust to 1600 rpm if it's incorrect. This adjustment is carried out by turning the screw in the centre of the fast idle diaphragm device on the carb. The fast idle diaphragm device is the lower of 2 on one side of the carb and is connected by a link to the primary barrel throttle plate, (i.e. it acts to prevent the throttle plate /butterfly valve from closing completely, thereby raising the idle speed). The fast idle speed doesn't usually need any adjustment.
The above info is for a carb. model, which I assume your car being a '78 is.
Al.
