All 6 bbls feed idle fuel, and idle fuel flows at 2600 rpm, actually it flows to some degree at all rpm. I would guess with the rear carb idle screws being closed that is a big part of your lean problem. I would try all the outboards at about a 1/2 turn and see if it helps the lean problem. I don't think you want no changes when you cover outboard idle bleeds, if you cover them and it should get rich and lose a few rpm if its right in my opinion. It is possible the rear metering plate idle gas feeds are the wrong size, or blocked, could be blocked or restricted in center metering plate also. You need to get all 6bbls helping with the idle, and it seems best with outboards at 1/4-1/2 turn, center at 1 to 1-1/2, actual mixture, idle air to fuel ratio is done with idle gas feeds and bleeds, more the gas feed than bleeds as drilling those is a good way to ruin a carb. Some of these newer 6bbl carbs are a nightmare.