compression is still 110. Rings are right way up.

Interesting point though on the ports, maybe I'll pull the head today and take a look.

 

If it has run for a while and compression is good, I don't think the rings have "caught" so probably never will. My guess would still be carb, unless something drastic has occurred in the exhaust. I once got a Suzuki 50cc two wheeler to repair which suddenly lost all power, took the exhaust off, it became unbelievably noisy for it's size, but would now pop wheelies. Found a baffle in the silencer had come loose and blocked it.

 

With the correct air/gas ratio, compression, and spark at the right time...it has to run, right?
So...
You"re comfortable that the carb is clean and I'm going to assume that means you've also checked the air filter and it's getting air.
Is the float bowl/needle functioning correctly (i.e., is it flooding?)
It has spark, but the timing could be off? (Not likely since it was running before, unless you did something to it).
Compression...like eyebrows there should be two. One is the piston compression stroke which you said is 110 psi. The second is the intake stroke which is compressing the mixture in the crank and forcing it up through the piston ports. Leaks around the crank seals (or reed, but you said that looked good) can be leaking and sucking in air and making it run lean.
This might be grasping at straws, but you can check it with a couple of rubber plugs.

When I was looking for a crank seal leak I pulled the carb and exhaust and fitted tightly fitting rubber plugs in each opening in the motor. You can probably clamp the one at the carb end with the hose clamp. The one at the exhaust may have to been strapped in place running a ratchet strap around the cylinder.
What I did was drill a hole in each plug. In one I threaded in a metal air valve; in the other I threaded in an air gauge. With everything plugged, take a bicycle pump and pump up 5 or 6 pounds on the air gauge, then watch it to see if it bleeds off. (You can be spraying soapy water on it too just to see if your plugs are sound). The pressure should hold for several minutes.

This may sound kinda half/assed, but it worked for me. Just don't pump up too much pressure and blow a seal out.

Merryman...feel free to critique this method. I'd kinda like to know your opinion. Be gentle

Another good point Merryman made was the exhaust. I've had dirt dobbers build nests in the exhaust and tore the carb apart twice before I found it.

...and for the old, old farts in the crowd, anyone remember the old Can-Ams bikes with the exhaust stinger that pointed down? Drop the bike on that side in the mud and it would plug the exhaust solid.

 

Not a bad idea anymore as they make starter fluid with upper cylinder protection...widely available...even before that I never popped a motor by firing it once for testing purposes...highly unlikely enough heat would ever be generated in a few seconds to seize anything...ring gap and all...it's an effective diagnostic tool and wouldn't discourage anyone from using it...

 

I've learned something. Thanks!

 

I had decent compression but wouldn't run also, I pulled the jug and replaced the head gasket and lower gasket. Runs great now. I recently learned that you can have compression but you need to do a leak down test to really get a good idea if it's rings, gasket....etc

If the cylinder setup fails gain, Im going to LT80 for some non Chinese parts