Carrier LP furnace gas valve problem

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 are:
- 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.
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Replaced by a pro and checked out completely
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You REALLY need to call in someone that knows what they are doing. A gas furnace is nothing the experiment with......
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This is Turtle.
By what you say the gas valve not closing properly when turned off by the thermostat. Well have the furnce running and then pull one of the 24 volt control wires off the valve to cut the power to it. Then if it slowly closes and holds a yellow flame for 5 seconds or so. You have a bad gas valve and the cure is to replace it and not repair. Repairing gas valves went out the door somewhere about the 1960's .
There is slow opening and closing gas valves but none will hold a yellow flame for 5 second like you say and they working good. I would suggest you call a service company to check it out and make sure it is the gas valve and may have something else too wrong causing this problem.
TURTLE
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