Hi diy,
From miles away, and not being able to see or hear your "popping", this sounds like this could be a typical Model A that did not run for extended periods in a long time.
If so, with this possible condition, the volatile components of the oil lubrication at the valve stems & valve guides evaporated and left a sticky varnish material such that the valves are sticking, & moving up and down slowly; hence valves could be out of time causing delayed combustion "outside" of combustion chambers which one can readily hear & sounds like "popping".
As stated, (again if this is the case), moving the spark lever up & down changes ignition timing slightly, affecting engine "popping"; however, ignition timing changes are not enough to correct delayed valve timing, "and popping".
This "popping" caused by sticky valves was as common as a common cold in our area where old vintage tractors were not used in off-season-periods ...... sounded like a 12 gauge dove hunt in a farmer's back yard.
Before Liquid Wrench, Marvel Oil, & other "Mechanics in a Can", we all soaked our valves for 24 hours with Kerosene poured in each spark plug hole ....... this was the vintage miracle cure that worked 100% of the time to dissolve the varnish, set the valves free, and made our neighbors quit staring at our "popping" war zone.
But remember, ............. just one possibility.