Is the pan setting directly on the fire or is it setting on the stove top or is there a slight space? Somehoe the picture 3 of 3 looks like there is a space under the pan. It might just be an illusion.
It might help if you put some firebrick against the sides of the pan. Also, how deep is the sap? It looks like maybe the pan is 6" tall and the sap maybe 4" deep. If that i correct, you want to have less in the pan at any given time. 1" might be best but be sure you maintain that depth at all times. Maybe 1.25 or 1.5 might be safer but it will not boil as fast.
Back when I had a 2x3 on a half pint arch (the fire was in direct contact with the pan) I got 6.5 GPH when I had good wood, split wrist sized, filled the fire every 7 minutes and kept the pan at 1" deep. If I had to take a break I added another inch depth, that not only gave me a few minutes but slowed the evaporation rate significantly. (I do not recall exactly how much but it was at least cut by 50% if I recall.)
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.