Black soot from propane furnace.

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I have a coleman/evcon propane furnace and it seems every year I have to have it fixed every year and it's only about 9 yrs old.but the most recent problem is when the furnace lights every thing is ok until the blower kicks in then I can hear a click like its the gas valve and the burner goes out,it cycles through a few times and shuts down,I turn it off and back on again and it will run again a few times and ocassionally it will run for a day or two and then won't turn on again,I have just replaced the thermostat and it worked for a few days and quit again,I took the main burner out and checked the stack for obstructions and couldn't find any in the fresh air or exhaust I used compressed air and a shop vac and everything is open,also the combustion chamber has alot of black soot.

Any body have any thoughts
Thanks in advance
Rich

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Black soot is an indication that your burner is not operating correctly. Could be a partially restricted main orifice or the pressure regulator adjustment on the gas valve. It could also be a bad air over adjustment. Who ever you have been calling for service should have checked the valve and air adjustments during service. You need special tools to check the manifold/gas pressure leaving the gas valve. Black soot is very bad news on propane and should be attended to immediately. It could be part of your other problem with the on and off thing.

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