Flapper in heater fan, What does it do ? Options

[Olympic 914]

The heater fan motor was pretty cruddy so I disassembled it and polished the armature and greased the ends then bench tested it and it ran pretty good. loosened the bolts on the housing and while it was running wiggled it around a bit until it ran the smoothest. when I reassembled it I noticed a clear plastic flapper inside the blower housing. No spring on it to hold it closed. When I put the fan in the car with the outlets pointed in what looked to be the best direction, pointing towards the pass side, the flapper valve hung open.

Looked at the "originality photos" and see that the outlet was installed to point straight up (?) in this position the flapper closes by gravity and would open whenever the fan is blowing.

What does this flapper do ?

[rhodyguy]

Your car should have a single outlet fan with a splitter for the hoses if you want heat to both sides. The cooling fan pressure should close the little door as engine speed increases. I don't bother with a aux fan. I just cap the jtubes with a couple of 2"(?) rubber pipe caps from the hardware store.