My 1200 ute in PNG was run without a thermostat and was fine. I couldn't get a replacement quick enough and had to run without.
The engine I just removed from a friends recent swap also had the thermostat out and there wasn't an issue (good chance to plug the video!
Here) he also ran a thermo fan full time. Sure a bit grumpy at cold start but no hesitations or splutters.
Who tuned the carb and how was it done?
If I throw the pair of webers I have here on my engine it will rev hard and idle fine with about 10 minutes of tuning take it on the road and it will stall before I am out of my street. A perfectly tuned carb means it has been tuned and assessed off load and under load, under load meaning driving it. Have you had this on a dyno? Or have you done a road tune with a sensor.
Finding it very hard to believe it could be much else unless you have some other obscure problem.
Do you want a running A12? Or would you like me to send you a carb off one of the two runners that I have here? You can try them and if the problem persists at least you have saved yourself throwing more money away at assessing the carburetor.
I have two motors, both were running and I can vouch for having driven one of them.