Thanks for the tip
Mike, since the AED had fixed air and fuel settings quite a bit of its performance is reliant on the engine condition, state of tune and the ambient temperature. I tried a range of temperature switches all the way to 60C at your suggestion but for my conditions and car the AED remained on to long. There is most likely some +/- variation in the accuracy of these snap switches.
…also a lot of difference between the tune of the main carburetors. When I bought my XK140 it started and ran fine without the hisser coming on at all. The otter switch was bad and someone had adjusted the main carburetors so rich that it would start without additional enrichment. The down-side was that it got 10 miles-per-gallon and blew a lot of smoke.
By using an exhaust analyzer and adjusting the carbs to 3.5% CO, where it ran perfectly at the proper temperature, I found that the engine needed to be warmer than 35° C in order to keep running. I found that 60° C was perfect for me. Of course, if your thermostat is missing or opens before 60° then the hisser would stay on longer or never go off.