car is getting 5-8 miles a gallon, black smoke out tail pipe, hard to start, on 1997 Mitsubishi Galant

1997 mitsubishi galant 165,000. Black smoke out tail pipe, hard to start when it sit for a while, 5-8 miles a gallon, exhaust smell up front. Have done fuel injection flush, changed spark plugs, change idle control value. What else can I do????

change the head gasket and take the heads to machine shop to be test it for worn valves seals