All good advice so far. Supercooled droplets in temps below freezing are almost certainly going to be visible. i.e. cloud and therefore airframe icing conditions exist, so you definitely should not be there in a Robbie.
In clear air at sub zero temps carb icing is unlikely, even with no carb heat. However, if you fly into a mass of warmer, damp air with your cold carb you are asking for carb icing. When I flew Robbies, on this basis I would always try to stay on the right side of the yellow arc. Headsethair is correct about the rich cut which will give very little warning before the engine stops. Trevor Thoms air law & met book has a good section on icing.
Better still, fly fuel injected.