Port 8091 is just one of the ports that must be open for Couchbase to function. You can see a list of all ports and what they’re used for here:
http://docs.couchbase.com/admin/admin/Install/install-networkPorts.html
In particular, you’ll need the “Node To Client” ports opened. However, you’ll also want to make sure Node To Node is opened between the nodes in your cluster.