Idle rpms

[Mercedes Fred]

a voltometer test on me 88 300e charging system has revealed a slight variance of charge at idle: the amps jump around between 14.4 - 14.7 at idle and smooths out to a constant at higher revs. the mechanic suggested the idle was varying, possibly due to a clogged injector, thus explaining the variation. Observing the tachometer, it reads approx 700 rpm at idle and barely varies noticably. I assume the voltometer to give more accurate readings than a dashboard tach.

Being of unsound mind regarding mechanical principles, I am wondering if the idle rpm "should" remain absolutely constant or if a variance is tolerable depending on engine age etc. and the %'age of this variance. perhaps i do indeed have a clogged injector, sticking valves or a deteriorating wiring harness...etc.