82 240d intake manifold help

when I pulled the air cleaner to remove linkage for a valve adjustment, I noticed a butterfly flap inside the intake manifold that can be adjusted to open or close. mine stay mostly closed until the thottle was down about 3/4 of the way and then just opened 1/4 of they way at full throttle. when and how far should this open for the best perfomance. my car is really slow but I've been getting it better with lots of tuning but still needs all the help I can get.