Yamaha 9.9 new model idle

A sailing club I belong to has 6 Yamaha 9.9 outboards powering out 27' Catalinas. We have had 2 of the new model engines develop rough to no idle conditions requiring service shop repairs to correct. First problem occurred 2 years ago and we installed a pre filter system and have had no repeat problems with this motor. This may just be a coincidence or is there something about the new model design ie smaller ports for efficiency etc that is causing this? Is a pre filter necessary for the new models? Anyone else seen a similar pattern?

The carburetors have very small jets and plug easily if fuel is left in them for prolonged periods.----Then again you may need to adjust idle mixtures.------------2 or 4 stroke ??????????/

you need to keep the filter and adjust the idle speed up