My older (20+), Carrier, gas furnace has three burners that light normally. The left burner?s flame changes from normal focused flame into a soft billowy flame after about 16 seconds. The flame then spills out the front of the heat chamber as it struggles to find oxygen. Gas is distributed to the (3) burners by a single pipe with three ports. We cleaned the heat chamber of lots to rusted flakes, but the problem continues. Have you seen this condition corrected other than total replacement of the furnace?

Sounds like you may have a/some holes in heat exchanger, which is not good/safe the heat exchanger can be replaced but very expensive, probably better off to replace furnace

Rust from the heat exchanger is never a good sign. I would certainly get it checked out by a pro, and look towards complete furnace replacement.