For my shop heat I have been using an old Rheem furnace with a Beckett burner running a CKBurner preheat / siphon system and a homemade kagi retention head. I burn used motor oil and have ran this system since 2012 with no significant ignition problems until 2017.
Last year I started experiencing problems with startup ignition. The burner would ignite 3-4 times in rapid succession, giving puff backs, before reaching a steady burn. When this occurred in Jan 2017 I replaced the nozzle o-ring, and subsequently, the nozzle. I saw very little, if any, improvement with these changes but managed to limp through the remainder of the heating season. Over the summer I cleaned everything and checked settings. When I fired the furnace this fall, it started and ran smoothly This lasted for a month or so, then the faltering start and puff back started again. At first, after a few minutes of puff backs with a lot of smoke which fills my shop, the burning would smooth out and then burn properly until the next start where it would start all over again. While trouble shooting the problem a couple days ago I tried clean #2 fuel and the burner started and ran smoothly but when I went back to used oil the puff backs reoccurred. I tried used oil from a different tank / source, same problem.
Yesterday the system ceased to achieve any ignition. When the system reaches the proper ignition temperature, the burner system will start the ignition sequence but all I get is steady smoke rolling out of the inspection port. There are no puff backs at this point. This failure to ignite happens even when I again try clean #2 oil. I have cleaned, inspected, and checked spacing of the probes with no change. I fired the probes outside the chamber and it appears there is adequate spark. I have tried a different transformer, different nozzle, adjusted combustion air, adjusted siphon air pressure, and different preheat and damper settings with no luck. I stuck my hand into the firebox in front of the nozzle (without an active ignition source, of course) and my hand gets oily.
Could the depth of the firebox cause this problem? The Rheem firebox is probably minimum ideal depth even in the OEM configuration. Last year to avoid continued liner burn through, even with a stainless liner, I modified the firebox by adding a firebrick target to the back of the firebox, reducing the depth to at most 11 inches from the burner. Looking at my furnace log, it appears this was about the same time the problem originally started.
If the firebox depth is the problem, why do you think the system started the season with no obvious problems and why has the problem continually gotten worse until reaching the point of total failure?
I am at a complete loss as where to go from here. Any suggestions?