The alternator should able maintain at least 13,6V output voltage at idle when
the headlights on, but the idle RPM must be at least 780 (800-820 recommended
for carburetted models). Below this RPM the alternator output voltage will collapse
rapidly (it can cause the unstable gauge readings and glowing charge fault light).
You can't rely on the 33's rev counter at this low RPM range, a digital rev counter
or an oscilloscope can be used to determine the true idle RPM.
The desired ignition frequency on idle is 13 to 14 Hz, measured between the ignition
coil negative terminal, (marked with "1" on Bosch coil) and the chassis.
Use at least 10V/div vertical resolution. If available, you can use an inductive pickup
as signal source on the ignition coil high voltage output cable.
Good luck for the nephews