Mac, I'll try to help you get to a solution to your problems. First the AHS Coalgun is a good boiler and properly installed and operated will give good long term use. It along withe the A-A boiler is unique in the way it burns coal. Coal is burned in a tube, hence the term "Anthratube". When this method of burning coal was researched in the early 1940's there was nothing like it, in fact some of the experts at the time were surprised by the test results. A report was published in 1945 on the Anthratube. I've uploaded a copy. Much of the development work was done by Anthracite Industries, Inc. A company long gone. The report is somewhat technical but what I want you to understand is the concept of "fire height". By this I mean where the coal actually burns. This is important because if the "fire height" is in the wrong place problems result. The only way to control "fire height" is with the ashing grate. Stop the grate operating and the fire will burn higher. Stop it too long and you will have the fire burn it's way up the feed tube and you will have a hopper fire. Operate the ashing grate more and the "fire height" will be lower. Operate it too much and you will put burning coal in the ash bucket. Followed by raw coal.
So between these two extremes is where the "Coalgun" needs to operate. If you read the referenced report carefully you will see they determined the fundamental bed burning properties for Anthracite.Anthratube_Design_History.pdf
I'll quote from page 59 of the referenced report.
"As a result of the investigation of the fundamental properties of beds of burning Anthracite, it was shown that the thickness of the oxidation or intense burning zone in the bed depends depends upon the size of the fuel used. In general, it was found that Anthracite burns approximately 4 to 5 pieces deep. In other works, the oxygen in the air for combustion will pass a given amount of fuel surface before it is completely consumed."
Read and re-read this until you understand what it's saying and then think about the implications. First the burning coal is only 4 to 5 pieces deep, not much using Anthracite pea coal. Second all the oxygen in the combustion air sucked up by the combustion blower gets all used up. This is key to understanding "puff backs".
Let's think what happens when the "fire height" is just right, whatever that is. When the blower is running hot combustion gases are drawn into the boiler tubes heating the boiler water. Sufficient air flows thru the coal bed to support coal combustion. Next the blower stops. The inspection port opens. Fresh air with new oxygen enters, drawn in by both chimney draft and hot air rises principles. IF, the "fire height" was such that all the oxygen was used up AND combustion-able gases exist a "puff back" will occur. The oxygen for the "puff back" came in via the inspection port door. This is why minor "puff backs" are often heard when the combustion blower turns off.
So, what's the correct "fire height"? Seems to me it needs to be no lower than a few thickness of coal below the bottom of the inspection port. You should be able to see the burning coal when you look in thru the inspection port. If you do it means the coal combustion gases that didn't have enough oxygen to burn are on their way thru the boiler tubes. Sucked there by the combustion blower. Now when the blower stops and fresh air enters these unburnt gases are past a source of a flame which would ignite them. Adding fresh air has no effect and the open inspection port door just allows the natural chimney draft to work.
How to adjust "fire height"? It's changed by operating or stopping the ashing crank motor. It has nothing to do with the thermocouple controller temperature settings. The controller settings are used to maintain the position of the "fire height" oncee you get it to where it needs to be. Raising the "fire height" can be achieved by breaking the electrical connection to the grate motor. Clearly if it's not ashing the "fire height" will rise up in the "Anthratube". Conversely, applying power to the ashing motor will lower it. How you break or make the connections to the motor is up to you. I chose to put the motor in series with a switch. Normally the switch is closed. When I want to raise the "fire height" I open it. Since coal responds slowly you need to watch it, because if you forget it, you will have a hopper fire. Use caution.
You could use your thermocouple controller as an on-off switch. Just use extreme set points that either turn on or off the grate motor. Remember these are temporary set points only. Used solely for setting initial "fire height". Again pay attention. A wrong setting for too long could cause a hopper fire or an out of fire condition.
I hope this helps.