Poor compression in one cylander at idle.

One other test you can do is to pressurize bad cylinder with 100# of air when both valves are closed at BDC after backing off adjustment, and listen in intake and exhaust for air leaking. Then gently tap on top of rocker over both valves one at a time with hammer listening for escaping air and ability of valve to seal up. If head holds pressure, than you know that there is a ring problem.

As others have said you could have stuck compression rings from buildup of carbon, or broken ring. Sometimes you may get lucky with using ‘mechanic in a can’ snake oil treatment that promises to free stuck rings, improve compression, stop smoking, cure leaking body parts, and bring world peace, and sometimes not.