Originally Posted by
Crash one
If ice reduces the air flow wouldn't that also reduce the amount of fuel sucked in by the air? Therefore keeping the mixture ratio about the same?
It depends on the particular carb. Most carbs use the airflow velocity or the aiflow pressure (actually the pressure
drop) to meter the fuel through the jets. As the ice constricts the flow the air mass flow through the carb drops, but the velocity increases. As the constriction is in the venturi (downstream of the butterfly) the fuel metering "thinks" this means the mass-flow is increasing and so provides more fuel (making the mixture rich).
In a pressure-driven carb the increased velocity increases the pressure drop (manifold depression/vacuum) which the carb "sees" as addtional flow due to an increased throttle setting, so (again) it supplies more fuel and enriches the mixture. If the ice ever did get to block the main jets then you might see a weakening of the mixture, but by that stage the engine would proably be barely running (that's a guess on my part)
PDR