Here is an update. I changed the plugs and #8 plug seemed to be different than others (wet). New plugs didn't change anything. Then I checked spark on each cylinder using spark gap tool. No spark on #8 cylinder but nice long blue spark on #5. Also pulling #8 wire made no difference on idle roughness unlike pulling the others.
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I checked resistance on wires 5/8 1/6 2/3 4/7 and all measured 8.1-8.3 kOhm so there is an electrical connection from #8 plug to 5/8 coil. I'm aware of a high voltage there so my measure doesn't eliminate connection as a posiible cause.
Also I read a different opinions whether it's possible or not to have no spark on only one cylinder caused by the coil in a wasted spark system. Any experience with that?
What brand poduced yellow coils for ZR1?
Now I'm preparing for plenum pull, already got new coils and wires prepared and new gaskets from Jerry are on the way to Poland.