It has a 368 4\6\8 engine. It never seems to run on 6 it's either 4 or 8. Any ideas? Also if you push the gas more than half way it seems to miss real bad and blows black out of the exaust. I repalced the plugs 2 days ago with AC DELCOs it didn't help. About half the plugs were covered in soot. The car has 71k on it and doesn't leak anything. Any help would be great.