These do not drop voltage to 9 volts. They have a reverse breakover voltage of close to 9 volts. 

500mW DO-35 Zener Diode used in many Honda motorcycle ignition systems. Without it there will be no spark. Most aftermarket ignition modules do not have this diode built in so you will have to add it. You can either solder it onto the empty contacts of the replacement ignition module or solder it into the wire, shown as the pink wire in the image above that runs to the ecu. It is directional so it will need to be installed in the proper direction to work, but no damage should be done if installed backwards, it will just not work. This diode also acts as an anti theft deterrent.

I do not have a complete listing of what models need this diode, only a partial. If you have additional models to add, please let me know. 

7/13/2023 - just fyi

I have had a few people (2 so far) that think these are used to drop voltage to achieve the intended goal. While they do drop forward voltage a little (approx 1.6 volts), that is not there intended purpose in this application. They are used here to prevent current flow up until the breakover point (approx 9 volts) where then reverse current is allowed to flow. I found a nice link that explains this more from the university of Oklahoma. See "zener diode section" . 

https://www.nhn.ou.edu/~bumm/ELAB/Lect_Notes/diodes_lecture_v1_2_2.htm

I hope this helps.

Thanks for looking