I have a 20-year old Carrier 58SS furnace with a 646A-X gas valve
converted for LP. Beginning at the point in the heating cycle when
main burners are burning, the symptoms a

- When the thermostat is satisfied, the main burners turn almost off,
but continue to burn for about 5 seconds with much reduced vigor, and
with 2-4 inch orange flames burning vertically from the orifices.

- the pilot continues to burn, but also with a much smaller flame.

- later, when the thermostat calls for heat, the main burners never
light off.

Troubleshooting revealed the following:

- if the gas supply is turned off, the pilot, of course, goes out,
and if the gas supply is turned back on, the pilot hisses.

Workaround is:

- when the thermostat is satisfied, and the main burners go nearly
out, I turn off the gas supply and the thermostat.

- when I want heat, I turn on the thermostat, the igniter starts
sparking, and I turn on the gas supply. The pilot ignites, and in
less than a minute, the main burners ignite normally.

I speculate that the gas valve is not closing completely when the
thermostat is satisfied, and if I'm correct, can the valve be fixed
(cleaned?), or must it be replaced?

Thanks.
--
bill
Theory don't mean squat if it don't work.