Fuel/air mixture

on my 87 camaro 5.0 my fuel air mixture is messed up how do you set or control this, when its running and after its warmed up I can close the primary flap just slightly and it runs smooth, if it is set to wide open like its suppose to be it runs rich you can smell the gas flooding and it starts to foul the plugs, so How can I adjust this.

thanks

Ira