100 psi on that engine sounds Low. I would redo the Compression Test with a different Gauge. Don't start sanding the Reeds, as that will result in you needing to find Replacements. They are Check Valves, and if there is a slight gap at rest will, 99.999+% of the time, will seal when crankcase pressure rises.
Can the Spark, jump a 3/8" Gap, on all Cylinders?
When you rebuilt the Carbs, did you use genuine Mercury/Quicksilver Parts?
How did you check/adjust the Float Levels?
A Serial Number would be nice, as there are 4 Groups of Serial #s for the XR6 and there are variations of the Carbs between them, and even within them.
It appears some have non adjustable Idle Mixture Circuits.
When you did the Link & Sync, where did you get the Specs for it?
