Why is your 400ex backfiring when you leave off the throttle?

Backfiring in a Honda 400EX when you let off the throttle is often caused by an overly rich fuel mixture, which can result from a dirty air filter, incorrect carburetor settings, or an exhaust leak. Additionally, it may be due to unburned fuel igniting in the exhaust system. Check the carburetor and adjust the air-fuel mixture, and inspect the exhaust for any leaks to resolve the issue. Regular maintenance can help prevent this problem from occurring.