Hi I have a wards furnace from the 40's I can't get it to stop short cycling I have changed the tromstat, thermo coupl, rewired half of the electcial due to wires being so old' cleaned the flut out, and changed the limit switch I can't get to stay running it wants to short cycle the filters are clean also I am woundering that the gas valve may be bad. I am getting about 19 to 20 volts from the transformer I don't know what else to do can anyone please help.