Gnob is mostly right. The weight of the flapper door would only affect the low flow characteristics of the flapper door's operation, like idle and off idle where the airflow is minimal. As to orientation..... With the flapper door orientated topwise your A/F mix will tend to richen as the weight of the flapper will be 'falling' helping the air to open it. Bottom orientation will lean the mix as the weigh will fight the airflow. The only orientation that should have any affect during any thing over off idle would be top orientation. When airflow gets to a point where enough air is pushing on the flapper, like putting around town or cruising speed, the flapper's weight would have no affect. Best orientation would be with the flapper down. As for stock applications, I have never seen the AFM in any position other than horizontal in 60 years (that I know of) of the Bosch style AFM. Yeah, the fuel injection used in our Toyotas is Bosch, if you didn't already know.
