It's a forced-air system, and has run well over the years. I've recently replaced the filter and the nozzle. The burner has a two-line (return) system. It's one of a pair of furnaces in the house, and the other one works fine.

The symptom is that is sputters/bucks/pops. What I mean is, the flame goes out for anywhere from .5 to 1.5 seconds, and will only ever actually fire for 5 to 8 seconds, sometimes less... Almost half-on/half-off.

At one point this was bad enough that it tripped the reset button, which gave me the clue to change the filter and nozzle... Which apparently helped, but did not fix the situation.

I've re-replaced the filter, and paid very close attention to sealing, reasoning that it must be air in the lines. I don't know how to effectively test for the spark cutting out, but I did shut off the oil supply and run it for a few seconds to see if I could see the arc.

I can see it, but only faintly around the flame shield and through the blue glass. I can't say if when the burner shuts off (flame goes out) when running, if it's still firing or not...


So, the possibilities I see:

1) Air in the line
-- tightened the filter strongly, after greasing the rubber gasket(s) well.
-- none of the fittings have been removed, and as far as I know, there's be no trauma outdoors (underground tank)
-- With a return on it, it seems like it should take BIG bubbles to affect it. There are no visible leaks on the filter or lines

2) Spark is going out, for some reason
-- the electrodes look OK to me. They're set about 1/4" apart, about 1/4" above the nozzle and just a hair forward of the plane of the nozzle. They look to me like the diagrams I've seen online
-- I tried a wider (80 vs 70) nozzle. No change.
-- I read somewhere that "rapping" the primary control, if it was flaky, might reveal it to be the weak point. Not so far
-- The transformer(s) don't seem unusually hot or anything odd about them


Ideas for the next step?

Thanks in advance!