Modulation and Digital Cable with Comcast

[<tom>]

I have Comcast Digital Cable for my home. The box is a Motorola  DCT-2000.
I also have a Channel Plus 5445 (4-channel modulator)

I am able to inject 4 channels into my home signal but I have a few
questions / problems.

Depending on what channel I inject to ( 77, 79, 81, 83), I lose some
of my digital channels.  The channel displays something similar to ,
"signal unavailable should be available soon".  If I disconnect my
modulator the channels appear fine.  If I choose different channels to
inject to, I lose different digital channels.

How can I find which channels are available for creating modulated channels?
Also, I'm unclear on Low Pass Filters
The ChannelPlus Low Pass Filter  LPF-750 passes CATV channels 2 to 116
Would this work?  Does Comcast use any channels higher than 116?

thanks,
tom