Minecraft wiki entry for Hopper_clocks, associated text: This clock uses a command block to slow the hopper transfer rate. The exact command will depend on the direction the clock is facing, but for clocks facing the positive X direction it will look something like this: blockdata ~2 ~ ~ {TransferCooldown:X}, where X is the number of game ticks (up to 32,767) to hold the item in the back hopper.