The acrid smoke appeared when I had the hotend at 225° I have never had the hotend up to that temperature before. I had previously been getting good prints at 195° and 200°. The hotend has always overshot the set temperature on initial heating. The hotend and bed have both been PID tuned. I keep returning to a problem with the thermistor, but everything seems to all right. I have tried a different thermistor. The fit is very tight so it should be secure.