You can only use a modem that is on Comcast's approved list. Comcast has to support the firmware and also various settings in the modem. Until it's tested and set up by them, it will not work. I think the 6141 is relatively new, so it's probably still being tested.
The Motorola SB6141 is listed as a supported modem on Comcasts site. The issue is that the bootfile for it isn't setup for automatic provisioning yet. What this means in a nut shell is your SB6141 may work but at best will get 25MBs down. To get faster speeds you will need to talk to someone who can either upload a new SB6141 bootfile or use the SB6121 bootfile. Both options need to be done manually so you'll need to get the right person on the line, most people in tech support aren't able to do this. There's a thread on it here: