The thermocouple works by having the junction of two dissimilar metals heated. When heated they generate that milli voltage. The safety coil in the redundant gas valve will be held in only when the milli voltage generated by the thermocouple matches it. You are bypassing that safety coil by depressing the button long enough for the milli volts to take your place. Typically what you are describing is one of three things: 1) Bad thermocouple 2) Insufficient heat on the thermocouple end...too small of flame or flame is so high it is actually bypassing where it needs to be 3) Defective holding coil in the safety circuit.
Why possibly a bad thermocouple? We are finding poor quality of products flooding the US market in virtually everything, why not thermocouples too... Usually if the junction of the two dissimilar metals (swelled up end) of the thermocouple is intact it will generate sufficient milli voltage, but that is no longer true with the lower quality of materials.
Though a slim possibility, the same applies to the redundant gas valves holding coil. If bad, the gas valve needs to be replaced. The easiest way to check it is with another new thermocouple. This will ring true ONLY if the pilot is adjusted properly and coming in contact with the thermocouple properly.
Make sure you have a strong pilot flame encompassing a minimun of 1/2 to 3/4" of the end of the thermocouple. There should be a nice inner core of blue with a different softer and larger outer ring of blue with NO yellow. Yellow or white in the flame will indicate imminent carbon build up. A tad of orange is all right as usually that will be superficial rust particulate sucking into the flame as you are or have been disturbing the area surrounding the pilot with all your work. That will go away mostly. Please keep us informed on what you do and what you find.
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Today's Featured Article - Madison's County - by Anthony West. Philip Madison has been a good friend of mine for quite some time. He has patiently suffered my incessant chit chat on the subject of tractors for longer than I care to remember, and on many occasions he has put himself out, dropped what ever it was he was doing, to come and lend a hand cranking handles, or loading a find onto a trailer. Although he himself has never actually owned or restored a tractor, he was always enthusiastic and always around helping with other peoples projects.
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