Mmmmmmmmmm.................
It sounds convincing but unforunately it's wrong.
The throttle butterfly is downstream from the venturi so the theory that the pressure in the venturi is depressd more at lower throttle openings is wrong, as is the evaporation theory. Pressure in the venturi is higher at low throttle settings, not lower.
When the throttle is wide open more air is passing through the venturi. Since the venturi is usually of a fixed size it has to travel faster, resulting in a greater pressure drop. It is this pressure drop that sucks more fuel out of the main jet so that you get more fuel to go with the extra air and hence more power.
The build up of ice at low throttle openings results from a number of factors:-
The throttle butterfly is acting as a large obstruction between the carburettor and the inlet manifold. Ice crystals formed by water vapour condensing out as a result of the pressure drop (remember how clouds form?) combined with the temperature drop hit this obstruction and stick to it, building up and blocking the airflow. In addition the pressure in the inlet manifold is even lower (otherwise the air/fuel mixture wouldn't pass through). The further expansion of the mixture as it passes through the narrow gap round the butterfly has a further cooling effect.
To exacerbate the situation still further the engine is generating less heat at the lower throttle setting, resulting in the hot air (if selected) being less hot and the ambient temperature inside the cowling being lower, thereby assisting in ice formation.
Last edited by Mike Cross; 17th Oct 2004 at 10:15.