The things that tend to cause the carbs to be rough at idle are heavy floats (they need to be weighed to rule these in or out even if you just did it 10 hours ago). The idle jet is clogged, but that should show up on a carb sync with a really high vacuum on the offending carb. An air leak around the rubber intake sockets for the carbs or at the air intake manifold where they screw down on top of the heads. Usually from someone taking these off and failing to make sure the "O" rings aren't pinched. This can and does cause detonation. Tweaking the mixture screw on the bottom by 1/4 turn should not cause a roughness. The 1.5 turns out is usually good for almost all people unless you live in an extreme condition. A choke being partly open and not closing all the way or a cable hanging up and not in sync with the other side.
Last week at CPS and the Heavy maint. Rotax class out of the way.