The Thor mixer valve is not a difficult thing to work with. Main thing I've found with the mixer is to make sure the needle valve tip is not damaged, and the packing around it is good enough to seal properly. That needle has a coarse thread on it, so its easy to turn it out too far and cause flooding or a no-start condition. You should also verify that the fuel passage in the mixer body is free and clear, otherwise fuel won't flow. Thors don't have any filter in the fuel system, other than maybe a brass screen in the tank. I suspect that a lot of Thor trouble that is attributable to the mixer is likely nothing more than a clogged or restricted fuel orifice.
Those engines also have crankcase bearings that are clamped in place by the two steel stampings that form the crankcase. The upper bearing in particular will wear, and when the clearance between bearing and crankshaft gets excessive, it will not run regardless of what variety of curse words you throw at it. Check this by attempting to rock the flywheel from side to side and observe the side play (if any).
getting back to the mixer, it is devoid of any adjustment except for the obvious needle. There's no float to coat, and no choke or throttle butterfly to worry about. If you rock the flywheel over and the poppet valve burps or snorts at you, its a good sign. If the poppet valve is silent, it might be stuck, or the spring might be broken. There is a large opening in the backside of those mixers where you can lift up on the poppet to see if it is stuck to the seat. Thor Hansen that you could just stick a pencil in that opening to free up the valve. I suppose you can stick anything you want in there, assuming you stay within the boundaries of good taste......
Hope this helps.