Got Shell 93 Octane and now barely idles 1973 Dart 340

Mufflers sound like loud thumping and the engine. Took a long time to start and wouldn't idle this morning. Only to run is at 1000 RPMs and playing with the throttle. It's almost sounding like if you ever crossed ignition wires. But that was not touched. When I cover the choke with my hand the idle goes up but the exhaust and engine still doesn't sound right. My concern is vacuum leak, water in the gas as this started happening 10 min. after filling up, but the mufflers are a loud thumping.