What causes backfiring on your 1998 Honda foreman 400?

One possible cause could be too much air in the air / gas mixture. Check rubber hoses from the carburetor for leaks or visible cracks, or adjust the carburetor screws for gas and air mixture. Another possibility is that you are using a wrong kind of spark plug that is too hot for the engine. Then it will act like a glow plug once the engine runs hot, and ignite the gas and air mixture at the wrong time. Check for the correct spark plug, clean the old one or replace if it is the wrong kind or looks dirty.