It seems many GM vehicles around 1970 had an recirculated-outside air door in the heater air conditioning system. The valve opens in two stages. In recirculate mode it opens just a bit, letting in just a little outside air. It uses this mode for all heating modes, on Buick 1970. The amount of opening is adjustable on Electra 1970 when everything is apart. How much should the recirculate mode open the door? 25%? Can't find info anywhere, the chassis service manual omitted this.
The chassis manual actually said in section 13-3 to adjust the door so it closes completely. Doing that resulted in about 20 m.m (3/4") of opening in recirculate mode for me. So lets hope this works
