I've done a couple KNS pistons before with pretty good luck, but ran into something yesterday. On a Zastava M92 (which was gassed pretty typically for AKs pre-KNS), the rifle would not work on anything but the full closed (i.e. Full gas) setting, and it was barely ejecting at that - in fact, sometimes it would fail to eject.  With the suppressor on it, it was great, but I would like to have a little more gas for shooting without the can.  

FWIW, I have a JMAC Customs 360 muzzle device on it.

Has anyone see such a "leaky" KNS before where it barely functions/malfunctions even on the "full gas" setting?

One of YouTube guys (I believe Klayco47)  did a thing on it, The added weight of the piston makes the gassing marginal without a can on

My AKSU is like that. Runs fine completely closed without a can. Won't cycle on suppressor settings without the can.

It looks like Jmac has a booster shell (their site says "blast shield") for their muzzle device. You may need to run it when the suppressor is off.

I would guess that it's mostly a lack of dwell time. Barely any barrel in front of the gas port.

IIRC the 5.56/5.45 KNS pistons have a smaller gas chamber inside the piston head and may be less "leaky" than the 7.62 piston. I believe KNS actually recommends a 7.62 piston on an extremely overgassed 5.56 gun due to the larger gas chamber....so perhaps the opposite would be applicable as well.

I know my old M92 PAP is gassed perfectly, even with a booster. It's the softest shooting 7.62 AK that I own.

My 13" Galil ACE in 5.56 shoots empties into the stratosphere, but yet won't run any setting on my KNS piston except fully closed also

I had a SLR 106  ( 5.56 ) that did the same thing. KNS wouldn’t work in it.