The anti dieseling solenoid should have nothing to do with your fuel consumption, so address that as a separate issue. Have a look through the sight glass of the fuel bowl with the engine idling. It should be level with the lottle dot in the center of the glass.

The anti dieseling solenoid cuts off the fuel supply to the idle circuit in your carb when the ignition is switched off. This has the effect of preventing unburned hydrocarbons from passing into the exhaust system, & therefore, into the atmosphere.
A side benefit is the fact that without any fuel being drawn into the cylinders, the glowing hot spots that sometimes exist in the combustion chambers can not cause that "running on" or "dieseling" that can happen when you turn it off.

On hot days, the trick is to turn off the air conditioner [where fitted] as this kills the idle-up device & allows the standard idle speed. Wait a couple of seconds to allow some degree of cool down, then switch off. No embarrasing banging, knocking & running on. I love 'em.