My 02 400ex is hard to start and when it does it idles a little and is responsive to the throttle but then dies. When it is idling there is a popping kinda of sucking noise coming from the carb area. The previous owner tweaked the card on a 5000 foot ride and now it is sea level and runs rough. Where are the air/fuel mixture screws at on the thing and any help for a good setting would appreciated.