96 Triumph Adventurer 900 Mileage Issues

Hi. I recently purchased a 1996 Triumph Adventurer 900cc that has sat for 12 years. 16k miles. Been working on it for a few months after extensive research for parts which I found. It has 3 mikuni BST 36 carbs. I had a so called tech clean the carbs three times. Still wasn’t running right. I took the carbs apart and the job was done poorly. He put new jet kits. Mileage was bad and black smoke out the exhaust. I cleaned the carbs thoroughly, replaced the float assemblies with new ones. Set the heights to spec along with new bowl gaskets. New head to carb rubber mounts. Also synced the carbs. It’s running great but only getting 15 mpg. Carbs look new inside. Wondering if anyone might have a clue as to be the issue. Any info would be greatly appreciated.
Thanks everyone,
Dan