Correct idle speed adjustment is essential to avoid stalling in traffic or an excessive thump on engagement of gear from 'N'. A high idling will cause excessive 'creep'.

With the engine at normal operating temperature and the selector in the 'N' position, set the idle speed to 750 rpm, or 650 rpm for models prior to 1974, using an accurate electric tachometer.

It is normal for the idle speed to drop 50 - 100 rev/min when a gear is selected.

Source: http://www.reizendemoke.be/techniek/manual/autotran/1.htm