I have a sears 600 wall unit and the upper fan stops working sometime I have put my hand on the blades and spin it to get it going and it make a burning smell and noise

Sounds like the fan motor is failing. Replace the heater or have it repaired by a qualified facility. You have two sources of fire, a burnt up motor and no dissipation of heat out of the unit. Don't use it unless you have it checked out.

If the fan motor has a capacitor, the fix may be as simple as replacing the capacitor.