The accelerator cable and linkage control the throttle butterfly. The engine demand, in conjunction with the throttle opening, control the opening of the air flow sensor plate. I believe the airflow sensor plate is what you are calling the throttle plate.
Presuming this to be true, the plate could be gummed up with deposits - clean with a rag and rubbing alcohol or such. Also, there could be a problem with the fuel distributor requiring its service or replacement. Adjustments will not affect this, and could really screw up your mixture, if you are not monitoring.
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7