I have a 1997 Honda Shadow Ace. Backfires at over half throttle.

I have a 1997 Honda Shadow Ace. Backfires at over half throttle.

I have a 1997 Honda Shadow Ace. Started to file plugs so I took it to a repair shop. They rebuilt carburetor. Now when it's over half throttle it backfires through the carburetor. They are convinced it is not a carburetor problem.