I have an update: I was able go get the FC
running pretty good. It's got a slight miss that I haven't tracked down yet (you can barely hear it toward the end of the video). I'm going to replace the plugs, points, condenser, rotor, and cap (if I can find them) in hopes that fixes the miss.
In my quest to find more about this welder I ran across this YouTube
video. The unit he is filming is an Airco Hornet Special 200 amp unit. This is the exact same set-up as what I have (it even has the Waukesha FC engine). The only thing difference is the paint color, face plate, and the carburetor.
With this new information, does anyone have manuals, parts lists, etc. for the Arico?
I did some general exterior cleaning, got several contacts cleaned up, and cleaned up the armature (as best I could without digging into it too deeply). Armature before:

After:
I have still not been able to find any manuals / parts lists on the P&H welding unit itself. I struck an arc, but I believe the carburetor linkage is not correct. The previous owner had put a push / pull cable on the engine throttle but somewhere along the way the linkage was replaced with the incorrect versions..
There is a pneumatic diaphragm that is controlled by a copper vacuum hose connected to the engine intake manifold (red arrows). The pneumatic device is connected to the engine throttle via mechanical linkage (yellow arrow):

The pneumatic device has a 4-way "cross": first opening to the intake manifold, second to the pneumatic diaphragm, third to a mechanical "vent valve", and the fourth goes into the welding head. The face plate under the normal-looking household plug (seen in the first photos I posted) says:
AUXILIARY POWER
80 VOLTS D.C.
FOR 110 W UNIVERSAL TOOLS
AND PHOTO FLOOD LAMPS
SET CONTROLS FOR MAXIMUM OUTPUT
OPEN VENT VALVE ON VACUUM CHAMBER
TO RUN ENGINE AT FULL GOVERNED SPEED
There is also a switch that has been defeated by a small wire wrapped around (I took it apart and cleaned it up and now there is continuity when it is actuated). The switch and 4-way pneumatic cross can be seen where the pneumatic cross is connected to the welding head::
Has anyone seen a pneumatic set-up like this? Can someone explain what this pneumatic system to me? What I can't figure out is how / why the welding head is connected to the pneumatic diaphragm. Thank you for any information,
Jack