A quick question re 1991 Tempstar Furnaces.

I notice the blue flames and things are working great but the exhaust duct is rather hot (compared with high-efficiency furnace).

It seems that the heat produced by the flames is not well extracted by the Heat exchangers, a problem with older furnaces.

Question: can one reduce the gas flow at the Honeywell Valve to reduce the flame a bit to be more "fuel" efficient?

Just a "crazy" idea but does this come close to "high-efficiency"?
The idea of high-efficiency is to extract as much heat as possible.