MOTORCYCLE MECHANICS ...I NEED HELP,PLEASE!!!!!?

I have a 1982 yamaha virago 750 and i just got it back from having the carbs cleaned and rebuilt and synced,new plugs,coils,and gas and new oil change,the bike spits and sputters and backfires when i go to ride it now, there was some kind of va or air thing on the left side by the handle bars that had duct tape half covering where the air goes IN and when i took it off it started to run really bad,it ran ok when i first got it home but it did back fire alot,the mechanic said it was set to burn rich so he adjusted it but then later he told me there was not air/fuel mix screw that it was run off a va system. Can anyone with good knowledge of this bike or others similar to it PLEASE help me!!!! He said the compression was good on both cylinders so im lost....PLEASE HELP!!!!!