Since firing up this boiler I've had continual problems with occasional "puffbacks", "bumps", "boiler farts" - usually occuring after a decent burn cycle about 3-5 minutes after the thermostatic draft door closed completely.
Tried switching to Blashack nut coal from mixed pea/nut "basement coal"
Leaving an open spot of burning coal to burn off gas
Banking fresh loads, left to right, front to back, middle only, sides only, etc
Limiting the max opening of the draft door
Changed baro damper draft setting
Small load vs full load
Loading on idling fire vs active fire
The solution was in front of me the whole time - open the over fire air control half way. Idea came from this thread 1st day with AA130 on line.... then... BOOM! reguarding the removable plug in the viewport of a AA boiler.
It's been almost a week without a "boiler fart" regardless of how, when, or which coal I load. This boiler seems to be very "tight" in that with the draft door closed no air is getting to the fire. 2nd option was a paperclip under the draft door but that really seemed like a patch more than a fix. Dad has the manometer to tweak his oil burner so I can't say for sure but draft does not seem to have been effected - CO alarm not going off, no smell, smoke gets pulled into the over fire air port consistently. Thinking that the over fire air allows coal gas to burned or diluted enough so it doesn't go "boom".
Again looks to be the case with coal that what works for one person doesn't necessarily work for everyone.