I don't understand what you mean by your " upright furnace ". If a burner is burning yellow, it's usually a sign of not enough oxygen, however if you have just started it up, it could also be a spider web in the venturi ( burner tube ). You should probably take the burner out & look at the inside of the burner tube or if it's big enough, run a small brush thru it. Spider webs sometimes can be hard to remove. Hope this helps.
Home of the Wayward Wind
Bogfrog & Mr. Turbo
If in life you stumble, make it part of the dance