Hello there, if you search out the xs650 society, they have gathered all the relevant info about anything you may wish to know about carbs. The guide is called 'xs650 garage carburetor guide'. I found it invaluable for retuning my carbs after I put velocity stacks on them.
Depending on the carbs you have you may not be able to adjust the needle setting. This means you only have the main jet and pilot jet to play with. It is a matter of trial and error. If your bike backfires you are running way too lean. Start by going up one or even two jet sizes on the mains. You will also need to synchronize your carbs, and make sure the timing is correct before you start.
Get the guide, it will walk you through everything you need to do.