I have a Trane xl 80 gas furnace. During start-up, I get a flame roll out for a couple of seconds then it seems to operate normally. I checked the exhast vent for a blockage. What else could be causing this and how can I check it out. Thanks for any help.

The heat exchanger could be paritally blocked, or gas pressure may be to high. I would start by checking gas pressures.

I agree, check gas pressure first.