Text in force at 25-153: from 25-0, effective 1 Feb 1965.
If induction system screens are used--
(a) Each screen must be upstream of the carburetor;
(b) No screen may be in any part of the induction system that is the only passage through which air can reach the engine, unless it can be deiced by heated air;
(c) No screen may be deiced by alcohol alone; and
(d) It must be impossible for fuel to strike any screen.
29 FR 18291 · Source