Hey,
Do those carbs have idle screws and limiter caps (WMH's) or are they the old style WH's. either way, there are several things that can cause it. #1, have you checked to see if all 6 holes are running at idle by removing the plugs leads one at a time using a pair of grounded pliers? Each one, when removed, should drop idle rpm the same amount. If you can remove one or more wires with no change in rpm, then that one or more isn't running, either due to no spark, no fuel, or too much fuel. Try that and let me know. MAKE SURE YOU USE A PAIR OF INSULATED PLIERS WITH A HEAVY GAUGE GROUND WIRE FROM THE PLIERS TO THE BLOCK, OTHERWISE ITS GONNA HURT LIKE HELL, AND MAY DAMAGE THE ELECTRONICS!!