Sounds like the autochoke isn't switching off fast enough, its the cylinder thing at the right side with 2 coolant hoses and an electrical connection. Check the plug gets 12v when the engine is cold, if not check the red temp sensor in the plastic flange at the front of the block passes current when the engine is cold.
I think the problem is more likely to be the coolant channel o'ring in the inlet manifold has degraded and blocked the flow of coolant to the carb. This is a common fault on old dubs, did it on mine and its been perfect ever since

Let me know if you have any other carb issues as I've probably had the same problem with mine

Do you live anywhere near Bracknell? If so I'll be happy to take a look for ya
