my 400ex backfires pretty bad when i first run it then it gets better i have a yoshimura pipe whats the most likely thing to be causing it to backfire

 

Has it always been this way, or did this just start recently? Could be a lean condition?

Possibly never properly jetted when the pipe was installed? Could be a clogged jet? A leak in the air filter or air intake boot? Etc.....

 

alright thanks will it hurt to drive it like this

 

If it's running lean, then yes. You don't want to run it lean, as you can burn valves and possibly a hole in the piston. Have someone with some experience take a look at the carb.