Heat exchanger sooting badly, new exchanger Jan. 2009 (old one cracked and soot blocked)
Jan. 2010 Flame rollout switch, found heat exchanger sooting to nearly blocked, removed and cleaned, installed new face plate and gasket, Dec. 2010 flame rollout sw. found exchanger sooting to nearly blocked, blewout with high pressure nitrogen bottle, Dec. 29th. Two weeks later rollout sw. blew out exchanger and a lot of soot.
Model Carrier 48SS-024, inlet propane gas pressure 13inches, regulator gas pressure 3.5inches, flame blue with yellow tips, the combustion or draft sound is not there.
New combustion wheel Jan. 2010.
Orfices are #47,
What could be the sooting issue, what am I overlooking

With it being propane are you sure it should be running a 3.5 manifold pressure? That is typical for natural gas. I believe the #47 orifice in the Carrier gas packs are the factory installed natural gas orifices and should be converted to the proper propane orifice # depending on what altitude you are at.