First thing that comes to mind, when was the last time the cooling system was flushed? Second, if the present owner had to install a manual fan switch then obviously the stock switch is defective (thermostat type). There used to be a lower temperature switch setup that I saw occasionally on ebay. Also, the stock radiator is 27 years old and probably plugged up especially if the cooling system was not flushed on a regular basis, like every 4 years or something like that. You can buy a brand new aluminum radiator for the Venture for around $200. When you flush out your system, just make sure you are using products compatible with aluminum.
Lastly, if your carbs are out of sync that will make the engine run hotter than normal. Syncing your carbs should be an annual event...
PS replace your thermostat!