White smoke from exhaust = Burning oil
Black smoke from exhaust = Running Rich
If its not fouling plugs with oil then its valve seats. if it is then u need a ring job + valve seats
If your air filter turns black it means your engine is suffering from blow back.
Toyota 4a is a great engine and it is very hard to break down. Diagnose properly, our mechanics love to perform overhauls even when it is not required.
If your car is just running rich, check fuel system and wiring.