Engine roughness at low idle

The 'belleville washers '  are called 'disc spring' in the parts manual.

There are 2 or 3..depending if there is a clucth or not

in yellow in the attached drawing

https://en.wikipedia.org/wiki/Belleville_washer

The large metal bellville washers or compression disc if you like are down low in the gearbox. You most likely have 3 of them unless you have a 912UL and or older engine. The plastic thrust washer is up under the top small gear. Over time the bellville disc can loose tension and or the thrust washer could get damaged. The latter is fairly rare. These both get replaced during a gearbox inspection. If the vibration in the gearbox has gone on long enough or has been severe enough you can damage the gearsets and may need replacement during the inspection. Idling at too low an rpm, carbs way out of balance or a prop way out of balance will add and accelerate gearbox wear / damage.

What are the results of your latest friction-torque test?