Hi A10 JWO,
Testing with a Fluke it will /should behave just like a normal diode, OL when connected one way and showing 0.4- 0.6v
the other
To test properly you need a variable DC supply
Fit it to the bike and measure the battery voltage with the engine running, on revving the engine the voltage should not exceed 14.7v
Old Zener's go dodgy and the voltage threshold rises with age
A modern rectifier /regulator is a better option in this day and age
John