furnace wont run with dust cap on

[steven2themax]

I can get my furnace to run if I leave the cover for the pilot off (yes it does shoot 3" of flame out of the hole). When I cover it up it will run for a little bit but then the flame and pilot will go out. Any ideas?

[GeocamperAS]

The pilot heats a thermocouple. The heated thermocouple keeps the pilot gas flowing as well as allowing the burner gas to flow when the wall thermostat calls for it.

The first thing I would suspect is a dirty pilot. If any soot has built up, this would cause a weaker pilot flame. So when the burner flame comes on the pilot flame either goes out or becomes so weak, that the thermocouple cools down and closes the gas valve.
Use a toothbrush and clean the pilot.
If that does not solve the problem I would replace the thermocouple.
Hope this helps.

[ck21084]

Could it be that the fresh air intake is blocked thus sufficating the flames?

[exthemius]

You might also try the pilot orifice......we had the same thing happen to ours "after" I cleaned it (the holes are tiny, and it doesn't take much to clog them up). You can replace it for $5.00, and my pilot has not gone out since.

SUBURBAN PILOT ORIFICE Model: 160659