Can a 400 pound man ride a motorcycle?

Yes, but you might want to insure the bike's frame is stout enough and the motor strong enough to support the weight and move along and come to speed safely. Check the owner's manuals for proper weight limits and especially to insure the tires are appropriately inflated. Most manufacturers have weight limits and two sets of tire inflation recommendations; one is generally for under 200 pounds and the other for over 200 pounds to the weight limit of the motorcycle. A 400 pound man would likely be limited by what gear he could carry along, and might not safely be able to transport a passenger. Hope this helps.