Water Heater Burner Valve

I replaced the tank on my water heater, everything works fine (pilot stays lit, burns well, cycles properly) EXCEPT when the main burner shuts off there is a small flame in the burner feed tube next to the control unit. This flame is right underneath the gas tube going to the pilot, inside the slotted air intake portion of the main tube. Apparently the valve that feeds the main burner is not closing all the way? I'm guessing I will have to (1)replace the entire control unit or (2)replace the brass screw-in block containing the valve - if anyone has any advice I'm all ears. (obviously I am NOT using the water heater until I get this corrected)

Thanks

1984 Fleetwood Tioga
Atwood water heater model G6A - 7SPK