The symptoms are not a good match for weak mixture. If you can hear it firing individual shots when it is running at governed speed, it is four stroking or eight stroking, which is more likely to be poor scavenging than lean mixture. However if you have not checked both of the intake tube O-rings, which are notorious for leaking on older Powertorques, you should do so.
A spitting from the intake pipe when you run it with the air cleaner removed would probably mean lean mixture. A popping or banging in the muffler might mean leaky piston rings or incorrect ignition timing. The timing is not adjustable, but you should check that the magnetic pickup has the correct gap from the flywheel, and is not loose or badly earthed.
If you can't rock the blade plate or move it radially even a small amount, the crankshaft bearing is probably OK.
My suggestion is that you may have moderately weak compression, and the only cause left that you haven't checked, is worn piston rings (excessive ring gap). That could cause leakage of partly burned gas into the muffler (with possible popping sounds). Power would be less than normal, and starting would be harder than normal. If it has leaky rings you need to fix it or it will score the piston and sludge everything up.
So, replace the one remaining O ring (the second one in the intake pipe) and check the ring gap. At that point you should have a good base engine. If it does not run properly at that point, the next thing to investigate is the carburetor - but I think leaky rings are a better fit for your symptoms.
