During cold start and warm-up the system operates in open loop mode and the mixture is on the rich side. Once the system goes into closed loop the idle mixture is maintained at stoichiometric, so if one or more injectors have a flow or spray pattern problem, the resulting leanness causes combustion instablity or intermittent misfires that cause idle roughness.
I notice on my 190E 2.6 that the idle is butter smooth - as should be the case for an inline six - until the temperature reaches about 60C. Then it picks up a slight roughness/uneveness until the next cold start. Sometimes the system will also "hunt" - slight cyclic variation in idle revs - until it finds stability. As I said before, a slight idle roughness seems to be in the genes of the KE system, and mine isn't bad enough to tear into the system to try and fix it, but I try to give it Italian tuneups as often as possible.
Pre emission cars with carburetors idled at an average A/F ratio of about 12.5:1 to maintain best idle quality. With port injection and well matched injectors and air flow, a leaner idle will usually be satisfactory, but running at stoichiometric - about 14.7:1 - will cause roughness unless the injectors are matched within a couple of percent and a long duration high energy inductive spark is required to prevent misfires.
Also, high overlap camshafts cause more exhaust gas residual that can increase roughness due to combustion instability without a very rich mixture. These 103 engines only pull about 15" manifold vacuum at 700, which indicates more than average overlap for an emission controlled engine, but fairly aggressive valve timing is required on a two valve engine to make the high specific output that is characteristic of this engine family.
Back in the fifties and sixties the nervous, lumpy idle of a special high performance Corvette engine was highly sought after and a mark of performance. The '64 - '65 versions of these engines with mechanical lifter camshafts only pulled 10" at 900-100 RPM idle speed, and the carbureted versions generally had better idle quality than those with the Rochester continuous flow mechanical fuel injection system that is very similar to the KE system in operating principle.