Since its been getting colder, you will see these post more also questions about pinging and timing. With the cooler weather the air is actually more dense, your engine is breathing deeper easier, which is a good thing. However with the increase in air density your air/fuel ratio goes a little leaner since your getting more air with the same amount of gas. Don't just add more fuel though to richen it up, cause the cold weather also makes it harder for the engine to burn the mixture as thoroughly resulting in poor mileage.
Adjust your timing, take out a degree or two, with the engine fully warmed up then see if it will shut off. Make sure your idle is set right and not to fast - as your engine goes lean idle speed increases. You may need to richen the mixture with your carbs, but go light it shouldn't take much. Check your plugs use them to tune the mixture in.
Some people call this dieseling, it's not just an SU problem it has many variables. Like on a high compression motor a vacumn leak can cause dieseling.
Thanks for the info Matt, my 72 has started doing this too as the weather gets colder. I tuned the carbs just yesterday but it still does it. Im starting to suspect leaky throttle shafts, they are dirty right where they enter the housing.
Yes, low octane gas or bad gas can be a cause. In colder winter temperatures you may notice worse gas mileage - because your engine cannot burn the mixture as thouroghly with the colder denser air and the motor itself running cooler efficiency goes down