Here's another thought. One time I bought a 125k mile slant six Duster that was "shot" "carboned up" "flooded" etc. It, too, continued to run even though the idle mixture screw was run in until seated. The problem turned out to be a bad fuel pump. The diaphragm leaked but the leak was internal so raw fuel ran into the crankcase. There was over 2 gallons of oil/fuel mixture in the crankcase and so much fuel vapor that the engine was running on it at idle through the PCV system. You may want to check for high oil level on the dipstick or pull the PCV while the engine is running, idle screws seated. The Duster ran great after that and I drove it for years. To almost 200k miles at which time the quarters were rusted to nothing. Pete in PA