Old school hustler fasttrak 48 kholer 19hp

Mower ran great last year. Muffler rotted off past down pipe. Ran 1 time with no exhaust cut greatvjust loud. Fast forward 5 months cranked and ran rough started dieing and only runs (although poorly on full choke) started getting worse and worse would run. I now replaced cheap carb. Coil and spark plug. Runs on full choke only. Can throttle up and down smoothly but when chole is.lowered or turned off it does instantly. Tried 1 and a half turns from full stop o. Carb adjustment. Tried 2 and half turns and everywjere on between best we can get is run but requires full choke! Last todbit, it also wont crank on its on accord regardless of adjustment screw settongs or choke it always needs a small puff of aeresol carb cleaner and then sje runs fine (on full choke)

Is it the lack of backpressure??? It ran fine last year without muffle can...

Did i burn a valve?? Is it simply 6 gallons of bad gas???

Help