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I know you haven't asked this but I would recommend changing all the bulbs on your motorbike to LED types:
https://www.dynamoregulatorconversions.com/led-headlamp-bulbs-shop.php
Not only will this give you far superior lighting but it will radically reduce the current load on the dynamo. It will also lighten the load on the bike's wiring, contacts, and switches. LED lights also last indefinitely in practical terms.
In addition, I would recommend that, rather fitting a Zener Diode, you fit a precision regulator, which will keep your battery properly charged and in good condition. One of us on ETO can do you a decent circuit.
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