Text in force at 23-63: from 23-51, effective 12 Mar 1996.
If induction system screens are used--
[(a) Each screen must be upstream of the carburetor or fuel injection system.]
(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--
(1) The available preheat is at least 100� F.; and
(2) The screen 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.
61 FR 5137 · Source