didn't occur to me before if there was no drop at all, the linkage might have come off
What about the gotcha when the shroud/box (not sure what the technical term is) has corroded/is defective to the extent that it is leaking in cold air, meaning that the carb heat effect is reduced - it is in fact supplying partial heat to the venturi. I suspect that theoretically, partial heat is worse than no heat as it may quite likely induce icing conditions with no options for a thaw!!