As almost everyone stated, check for a broken or out of adjustment electrode.
I would add that the flue could be blocked with a nest of some kind (spider or wasp) causing an over rich mixture at the burner inlet.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW