AJS & Matchless Owners Club

What state is the battery in? Are you sure you are testing correctly and the voltage drop is not caused by components in the system or poor connections to earth. It seems very unlikely to me that the wiring alone could be responsible for such a drop.

Yes, highly unlikely that the wires themselves are to blame.
It is very well known that all Chinese bikes are susceptible to poor quality and/or corroded connectors and switches which need to be given a good clean.
My Chinese bike had stood unused for 3 months when I needed to take it for an MOT last month. The indicators and stop light weren't working so I had to separate connectors in the loom and give them a clean with emery paper and squirt switch cleaner into the indicator switch and operate it 20 or 30 times to get the indicators to work. I always give connectors a light coating of vaseline before putting them back together.