Why do I have to unplug my datalogger / Pocketlogger in order to set/check my timing?

The reason is that the datalogger system grounds the connector in the diagnostic plug that sets the ECU to idle speed set mode, rather than timing check mode. In other words, when you ground the timing check connector with the datalogger plugged in, the ECU will go to idle speed set mode instead of timing check mode. Unplugging the logger will fix the problem and allow the ECU to enter timing check mode.