The issue you're describing with your Royal Enfield Thunderbird can be caused by various factors. It's important to note that diagnosing a mechanical issue without physically inspecting the motorcycle can be challenging. However, based on the information provided, there are a few possibilities to consider:
Electrical Problem: When you turn on the lights and the bike stalls, it could indicate an electrical issue. It's possible that there is a loose or faulty connection in the electrical system that is causing a disruption in power when the lights are turned on. This could lead to the engine shutting off.
Battery or Charging System: Another possibility is that the battery or the charging system is not functioning properly. If the battery is weak or not properly charging, it may not provide enough power to sustain the electrical load when the lights are turned on, resulting in the bike stalling.
Fuel Delivery: It's also worth checking the fuel delivery system. If there is a clogged fuel line, a malfunctioning fuel pump, or a dirty carburetor, it could result in fuel starvation and cause the bike to stall.
Ignition System: The issue could be related to the ignition system. A faulty ignition coil, spark plug, or ignition switch could lead to intermittent starting and stalling when the lights are turned on.
To determine the exact cause of the issue, it's recommended to have a qualified mechanic inspect your motorcycle. They can perform a comprehensive diagnostic check, test the various components, and identify the specific problem. This will help in accurately addressing the issue and carrying out the necessary repairs or replacements.