Yellow flames = spider webs. Whenever I get yellow flames on my propane burner, or BBQ grill, it means it's time to clean out the spider webs. The gas/air mixture usually travels down a tube leading to the burners. This tube seems to be a favorite spot for spider webs. Just a few is all it takes. Chase them out with a brush, pipecleaner, or rag. It never seems like it should be enough to make a difference, but it does. Blue flames should return, nice hot burn, quick boils, no black pots.