I have an old janitrol furnace the kind you have to ligh the pilot then turn the knob for tha main burners to come on. The problems is when I light the pilot it stays lit, then I turn the knob and all the burners light up and it goes for about 10 seconds then its like you went to the gas shut off ond shut it off, everything goes out, it never gets warm enough to kick the blower on, I've been through 3 thermocouples and I've taken the pilot line apart and blew it out and cleaned the orfice and nothing has changed

When the unit shuts off see if you still have 24 volts to the gas valve.