We're all agreed that carb ice is more likely at low throttle settings. I was just pointing out that the suggestion that the cause was a greater cooling effect in the venturi is wrong.
There is actually less cooling effect in the venturi at lower throttle settings.
At high throttle settings the pressure differential (and hence the airflow) across the venturi is higher and hence the cooling effect is higher.
With a low throttle setting the maximum pressure differential occurs across the throttle butterfly, not in the venturi and this is where the maximum cooling occurs.
This may be seen as a pedantic point but if the student has some grasp of the subject then trying to explain it by cooling within the venturi is likely to leave him (quite rightly) failing to grasp the reasoning behind the explanation.
My Luscombe has a C85 so I am well aware of the tendency to icing. I suspect one of the reasons small Continentals are prone to it is that the inlet manifold pipework hangs out in the breeze and receives no warming effect from the sump in comparison to Lycomings.
One other thing for the uninitiated. Most carburettors have an accelerator pump which squirts extra fuel in as the throttle is opened. This is to compensate for the lag between the extra airflow caused by the throttle being opened and additional fuel being supplied. The carburettors on small Continentals tend not to have this with the result that (a) The engine can falter if the throttle is opened quickly and (b) You can't prime by pumping the throttle.