Compression Loss on One Cylinder

I have been dealing with an idle/throttle issue with my KZ650. The engine will not always return to idle when the throttle is released, but the carbs are closed completely. I can load it down with releasing the clutch to lug motor, and it will stay at idle ok then.
I pulled the carbs to check them (again), and find no problems, so I decided to run a compression test. Turns out all cylinders are at about 135-145Psi except #2. It is about 50 psi. The engine runs fine at top end with no noticeable miss, & it does not burn any oil, or smoke. It is slightly rough on idle but not real bad.
Wondering what could cause the sudden loss of compression, but not burn oil or lose power? Could this be the problem with the return to idle?

With those kind of compression numbers the first thing I'd check would be valve clearances.

you can hook up a air hose to the compression tester and fill the cyl, make sure the valves are closed on that cyl, you will hear where the air is going either intake, exhaust, or in the bottom of the motor, but I would check the valves first

The Gringo wrote:

With those kind of compression numbers the first thing I'd check would be valve clearances.

Almost a guarantee that it's a valve problem. Either the clearance has gone to zero or the thing is gunked up and not seating. On mine, the valve seal went south and the oil seepage gradually gunked up the valve and seat until it didn't seal any more. Mine was down to about 80 PSI.