[Jim L]

With a 100 psi start and a 25% drop in diff pressure, the air should be audible somewhere. Set the tester up again and hold a smoke up in front of the carb inlet then the exhaust outlet. See the smoke moves at all. Then disconnect the case breather and do the same. It will be coming from one of these.

With your regular comp tester, pull both plugs and test both cyls cold and dry. Do #1 first and count the number of cycles to get the max compress value. Then do #2 using the same cycles.

Drop the piston to BDC and squirt eng oil into the cyl all the way around the piston. We are trying to put a coat on the entire ring's surface. Repeat comp test for both #1 and #2.

It will show up on one of these checks. Then you can merrily dive into its guts. I'm still suspicious about the rings. Tracks and rev limiters are very hard on valves, but a high reving 16,000 miles is tough on rings. Maybe it's both.
I'm curious to see the results.
Cheers,