Hello, I’m new here, but I have a 289 1965 mustang that I have used as a kinda daily drive, but recently we sent it to have the coolant system flushed and a thermostat replaced but later that afternoon something went wrong but the car died and wouldn’t hold idle. After turning the throttle stop up it stays running but when I put it in gear the rpm’s die super far down and act like it want’s to die on me, and did a few times. We just had the carb rebuilt so I can’t imagine it’d be that, but...

1965 mustang wanting to die when put in gear.