Preheat the grill on high. Scrape the grates clean. Turn all burners off except for one. You want to maintain your grilling temperature at 300 degrees F.
In a deep foil plan, top the pork shoulder with the sparkling water, orange and onion wedges.
Whisk together the brown sugar, chili powder, salt, cumin, oregano and cayenne pepper. Pour evenly over the pork shoulder.
Cover the pan with foil and slow roast on the side that the burner is not on (indirect heat) for 3 to 3 1/2 hours. Pork should be tender and cooked through - ready to shred.
Remove the pork from the pan and grill for a few minutes on each side to crisp up the exterior. Baste with the roasting liquid as you grill.
Remove the pork to a clean bowl. Cover loosely with foil.
Place about 1 cup of the cooking liquid into a saucepan. Bring the liquid to a boil and simmer for several minutes.
Shred the pork and pour the cooking liquid over it. Serve.