One of the biggest issues with these furnaces (and unfortunately common to combustioneer stoves as well) is cracking of the heat exchanger, once these cracks develop, it's very hard to keep them closed even after properly welding them or welding a plate over them because of where the stresses have been concentrated in the steel. I'm mentioning this first because they are preventable - if you don't have any, you'll want to make sure it stays that way. First the fan/limit control: set the limit at 160, close @ 110 and open @ 90. Next, you need more airflow! if it keeps hitting the limit either the blower is running too slow (larger pully needed on motor) or your ducts are way too small. Running two or even four eight inch ducts isn't adequate.
As far as adjusting the fire, you want to make sure of a few things: the clinker forms outside the firepot. the fire should be bright and NOT produce any smoke while running. as rockwood said, you want the fuel bed to be burning a bit above the tuyeres (extended idle times will reduce it's depth, but that's ok). You will achieve this by allowing loose ash to accumulate and by adjusting the air so that the fuel bed reaches an equilibrium (where fuel burned matches fuel introduced) at around 3-8" above the tuyeres (once again, the higher the coking tendency, the deeper the bed will need to be and the deeper the loose ash will neeed to be). You need to set your baro to between -.03 or -.04. you must have a baro (if you don't it will burn back into the worm and ruin it and the tuyeres). excessive air will also ruin the worm and the tuyeres quickly. If the clinker is forming as chunks in the firebox or as a ring outside the tuyeres, you are ok, if the clinker is forming in the tuyeres and you have a narrow column of flame shooting out of the center of the tuyeres (think raiders of the lost ark) you have too much air and the fire is burning down into the tuyeres. Some sparks are inevitable, but if it looks like a firework, you need to either increase the feed or (because these furnaces/stokers are underrated) more likely simply reduce the air. It takes some time for the stoker to visibly respond to changes either in feed or air; it may take a couple of hours or a day depending on load/runtime. The stoker will NOT behave predictably until the NECESSARY amount of loose ash has accumulated in the firebox - depending on the coking tendency of the coal it could be from 3 - 8" before the fire behaves consistently. Only remove the clinker ! Do not remove loose ash unless it becomes excessive or the weather is so mild that the stoker doesn't run long enough to produce a clinker - and then only take out what's necessary.
I'm sure there's a lot more but it's been a long day. Once you get it where you think you need it post some pics and we can critique your stoker fire