440 six pack high idle

Always adjust timing before the carb(s). It sounds like a carb issue for sure. Disconnect the outboards and I’d suggest taking a flat blade screwdriver and go down past the boosters on the outboards and tap the throttle plates to verify they’re closed. Then tune just off the center to get the idle you want. Once that’s set you can look at the outboards. I’ve had the gaskets hang one outboard open and the engine will run off just that.