Jd lawn tractor with Briggs and stratton 19HP engine. Engine pulsing or missing. Have replaced fuel filter checked fuel and air intakes in carby. Spark plug replaced still have issues. The rod that connects to the carby buttlerfly is going up and down on its own.

Usually due to an excessively lean condition. Most likely cause... varnish buildup on the jets from stale fuel (anything older than a few months). A vacuum leak can cause it too, but that's less often the cause.

Easy way to deal with it is use either BG44K or Seafoam mixed into the fuel... avoid most of the cheaper additives... they don't work as claimed.