Assuming you need to do this for maintenance:
1. Park the Honda 400ex on a level surface and turn off the engine.
2. Place a large pan or catch container under the engine on the left side.
3. Locate the drain bolt on the bottom of the left side of the engine. The bolt is located just above the exhaust pipe.
4. Use a wrench to loosen the drain bolt and allow the gas to drain into the container.
5. Once the gas has finished draining, use the wrench to tighten the drain bolt.
6. Remove the catch container and dispose of the gas properly.