Not building heat
No air in system and I can see coolant flowing through the radiator
If the radiator is flowing water when the engine is cold the thermostat is likely stuck open, blocked open, not seating propperly allowing coant to flow through the radiator.
As AJ/Formulas mentioned the thermostat sets the low temp.
If it is open the coolant will circulate in the radiator and cool the engine excessive.
Had that happen once here in Colorado in winter, drove 40 miles on the interstate, pulled into a gas station and the radiator hoses were barely warm, hence no heat or defrosters. I had a 10 inch opening on the fogged up window to see where I was going.
IMHO
Before you spend any time "fixing" the things you did not touch diagnose the problem to the likly things that you did touch.
- The fan will not be the problem at any speed over 25 to 30 mph.
- Water flowing in the radiator when the engine is cold suggests the thermostat is allowing coolant to flow cooling the engine too much
- Unless you changed other things when you changed the rad, the carb, tune, timing, choke etc are not the problem
