Maybe it's different on a 71 but on my 70 when I switch the air grabber off it closes pretty fast. If I leave it open and turn the engine off, the spring closes it but it takes an hour or so.

1970 Road Runner 505 cid MCH CNC ported Stealth heads MP 528 camshaft 4 speed GV overdrive 11.98 @ 117 on street treads