I have been working on a joule thief to act as a small 5 volt power supply and I have been using zener diodes clamp the voltage. My 5.1 volt zeners are regulating from 4.4 to 4.8 volts instead of 5.1. I tried other voltage zeners (9.1 volts, 6.8 volts, 4.2 volts) and they worked just fine. I have tried 3 different zeners from different sources (1 store bought and 2 salvaged from boards) and they all regulated to voltages lower than they should. Here is a schematic of the circuit.