Attwod water heater issue

I would start with the simple stuff first, remove the gas valve orifice and clean with alcohol. That hole is only about .020 in dia and can get clogged. Remove and clean burner tube check that flame spreader has not rusted off and is 90 degrees to tube. Does your refer run on propane? is the flame blue and steady or is it yellow and flicker? That will give you a good idea if you have a gas flow problem from the main tank through the regulator. If the slots in the burner tube are badly rusted that can allow too much air into the mix.