Rinnai EX38C Propane Direct vent heater Intermittent code 11

Going into the sixth winter with my unit. Started getting code 11 intermittently (every couple of days) It's become more consistent about 50% of starting tries. The lower flame rod and igniter are new, upper one cleaned. I have removed the burner cover and compressed air blown out some dead bugs.

The problem as I see it, (and I can actually see it thru the inspection glass) is that when it will not fully start and sets code, the front lower burner tube does not light (the one that heats the lower flame rod). The back one (at the igniter) lights but not the front. After repeated attempts, usually no more than two….the front lower will light and few seconds later both upper burner tubes light and every thing works normal until the next shutdown.

I've had the burner cover off to use compressed air but am thinking the flame tracks? are not getting fully cleaned. (Maybe rust or corrosion). Problem is I see no easy way to remove the burner tubes for deeper cleaning. Any help or other thoughts will be much appreciated.