Thanks Rindert.
This post by Kristijan describes the engine grade charcoal I should be using.
My simple fire doesn’t have a good way to shake it (no grate) and with my filter attached shaking the whole thing isn’t easy but I did notice that after I run it and would poke a rod down into the charcoal the level would drop a lot which sounds exactly what he descibed. I burnt the fines and left the bigger stuff.
That whole thread has some good information. Like this sketch from Tone.
Along with that skech, Tone says:
the difference between the upward method and the downward method is that in the former, all the water vapor rises during the generated gas and travels towards the engine, the charcoal dries completely and it is necessary to add water, well, in the downward process, a moist atmosphere is created above, which presses down through the hot zone, the steam is well superheated above and the conversion process is guaranteed.