Try driving out, reversing, and re-inserting the metal splitpins holding together the plastic couplers. I've found this takes some of the 'slop' out of the connection. Or, go to good auto or hardware store and buy new splitpins.
As mentioned in another post, synchronize them in the 'open' position, then set the back carb's idle screw so it just comes to rest against the carb body at idle.
Dick Sanders