I think David is spot on . If anything i learned in the oilsands , it’s all about temperature. It crazy how most won’t insulate when it accounts for about 30%
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I have 3" of rock wool and 2.5" of firebrick on the bottom and sides. I can place my hands on the side of the unit
are you pre heating ?
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Dave has a great point an evaporator will loose alot of heat from its function on the top anyway. If you have access to alot of sand you could try packing the outside with sand it would slow down the heat loss. Might make it start harder though. I keep thinking about masonary heaters they have a core of fire brick to hold the heat in the fire box a 1/4 expansion gap and then a outer brick wall for thermal mass. From what I have read about thermal mass walls I think it is 1 inch per hour as a general rule for heat to move through a normal brick. So that would slow the heat loss alot on a masonary heater.
Can you see the secondary air through the door? If so you should be able to tell if it is just burning slower then needed or not at all.
Hmm not that I would vote for not enough air or too little draft. What is the size of the riser at the back?
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The 1.5" hole you see above the door is my secondary air intake. It runs just under 4’ to a manifold I made and comes back to the front in 4- 1" square tubes. A bunch of 1/8" holes are drilled in each 1" tube.
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For a fire pit that big you will need at least a 6in.x 6in opening for a natural draft. That won’t be too big but if it is you can cover part of it with a plate. Have faith in me on this one. My outside boiler never worked because I had a 3 in opening with a blower. Then my furnace in the shop wasn’t working with about a 4in dia opening. I cut the shop opening to about 4x6 and it is much better but could be bigger. I have a slide plate that I can cover part of the opening with, but ususally run it wide open. TomC
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I didn’t realize it ran back without holes. That is a good pre heat maybe just needs more flow?
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Hight (and width) of the chimney + exhaust temperature = draft.
Only hot exhaust helps pulling.
Chimney insulation !!!
My 2c
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David, if you’re referring to the flue, it’s a 6"
too small of tube,is this 20 characters lol
Before you change it too much, try firing it without the blower attached. I still suspect you over-stoked it with the blower. Also I’m not sure if the impeller is blocking the flow with the blower off.
Another easy test, just leave the door cracked open. If that clears it up, you need more primary air.
Insulating the flue will have a massive effect. Think rocket stove.
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Just my informed opinion working oilsands and we use the most advanced reactors.,and scrubbers you want about a 5:1 air intake , preheated
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My wood gasification furnace from the 70s in the basement has a 8 inch exaust flue built into the furnace for a 100 000 btu rating and probably is burning half the fire size that boiler will burn.
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I agree with craig. My first boiler used a 6 inch tube and never burned well without the fan to assist. The new one has an 8 inch and pulls like a charm with only 8 ft of rise (insulated lengths though)
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BTW i help build the largest gasifier in North America . Opti Nexen . sadly they blew the plant apart
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Is your pan bottom at an angle?? if so, why? 
If it is that cold out, and the firebox hasn’t gotten warmed up, funny things are going to happen. I would put some snow or water in your pans and run it for several hours. Let it heat up to full temp which will probably take about an hour+. You will also get an idea of the evaporation rate as well.
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The pan wasn’t bent quite square and I’m sure the fuel barrel opening isn’t square either. I do have 2" angle iron around the perimeter for the pan to sit on to take care of the imperfections.
Today I ran it for about 4 hours, It did well without the fan after about the first half hour. The glass was able to stay clean and I was able to actually see the smoke being burned at the holes of the secondary air.
I put too much water in the pan, about 3", so I wasn’t able to get it to boil. When I put sap in it, I will keep the level around 1 1/2". It’s been 3 hours since shut down and the firebrick is still holding a lot of heat and the water is still evaporating.
Upon shut down, I plugged up all the air intakes. 10 minutes later, I looked and the secondary tubes were glowing red. I guess that’s why they use stainless steel.
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First time being able to collect sap. The wheeler had trouble getting up the hill with the mushy snow, could make the rest of the season a challenge. Either way, it’s still fun. The Maple trees are about 1/4 mile from the house.
I hope in 5 years to be able to do the chores using wood gas. Be it collecting syrup, processing wood or driving. Ir will make all my efforts worthwhile.
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Looking at the pic, are you asking if the pan is level? The whole evaporator was sitting at an angle in this picture. It was just a test fire so I didn’t worry about leveling it.