Griller:

85-90 grains of Goex FFG sounds about right with the maxiball. I wouldn't go over 100, at least not right away.

By any chance, do you know who the manufacturer is?

I'm sure this does not need to be said, but take the barrel out of the stock before giving the rifle a "bath." For some reason when I was reading Cayugad's (typically excellent) reply, I had this vision of you tossing the whole rifle in to the bath tub not just the barrel

You mentioned the rifle "clicked off." Were you using FFFFG in the pan? Flintlocks are kind of picky, they only shoot real black powder. Substitutes such as Pyrodex can be made to work, but in a hunting situation it is a real pain. Your best bet is to use real black powder only: FFG/FFFG down the pipe and FFFFG in the pan. DO NOT use FFFFG for your primary charge. The finer the grain (the more Fs), the higher the pressure. And barrels are not made for the kind of pressure FFFFG puts out.

Just some thoughts. Let us know how it works for you.
~Robert