Alan, a couple of thoughts: the purpose of the binary switch (a pressure switch) is to protect your compressor if the coolant level gets too low. The trinary switch has a second pole on the activated side to tell the cooling fan to run as long as the A/C compressor is commanded on. Wiring in a trinary switch, manual switch, and an engine coolant switch becomes a complex circuit, but is optimum. While your mechanic may be much more knowledgable than I, I do not agree that you want your carbed motor running cooler. It is also more efficient at higher temps. The max operating temps is determined by the lubricating media ie oil and the boiling point of your coolant. Our cooling systems are pressurized to increase the coolant boiling point so we now have the maximum pressure as our temprature limiting denominator. Modern engines are approaching the 22PSI level, but I will not go above 17 on my motors because weak points such as head gaskets, radiator tank seams, heater cores etc may not handle the higher pressure levels. Does not Walker provide a sensor bunge at the bottom of your rad? Many fan installation instructions suggest removing the drain coc for your fan sensor since you are dealing with your coolant temp. Now your fan would come on/go off when the coolant system temp is at its lowest (bottom of the rad)
Regardless, sounds like your fan sensor is not responding correctly on the low side.