You've proven that the top carburetor is functional.
You've moved the top carburetor to the bottom cylinder position with no improvement to the bottom cylinder which proves that the problem is not a carburetion one. You've also checked and replaced the reed (leaf) valve assemblies in an elimination process.
I assume that you've checked to make sure that fuel is indeed being supplied to that bottom carburetor. However, since you're sprayed a fuel mixture into the bottom carburetor throat with no improvement, that check is pretty much a moot one.
You say that the compression on all three cylinders checks out to be 115 psi... cylinders #1 & #2 are firing on that compression, it should be good enough for #3.
If a hole existed in the bottom cylinder crankcase area, I'm sure you would have noticed that... also, even if the bottom crankshaft seal was completely ripped out, fuel would still be supplied to the bottom cylinder in a sufficent quanity to have the cylinder fire properly.
If a engine has compression, fuel, and spark, it has to run... assuming the flywheel key isn't sheared, plug wires crossed, some other overlooked problem.
The one thing that I do not see mentioned anywhere in the above posts/replies is the word "spark".
On that model, with all spark plugs removed, at cranking speed, the spark should jump a 7/16" gap with a strong blue lightning like flame... a real SNAP! The 7/16" gap is important!
No tester?... See the following.
(Spark Tester - Home Made)
(J. Reeves)
You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:
A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.
Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:
..........X1..........X2
.................X..(grd)
..........X3..........X4
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