I have a carrier furnace that refuses to light. I light momentarily with a lazy flame. It eventually trips the temperature switch. I thought it may be the valve. Changed it, not it. Blew out all the air lines, no luck. Changed combustion motor. Still not happening. Took off parts and put them on another unit. All checked out fine. Pressure switch, new. Flame sensor, new. I don't like being a parts changer but this one has got me. Vent piping has been cleaned. Any ideas?