[Tom Goldman]

Rich is right on about removing the heads as opposed to using carbon cleaner.
Most carbon cleaners are a shadow of what they used to be before the EPA rules forced manufactures to tame them down or eliminate them completely, since the mostly used methlyene chloride.
My old Onan was so bad , I thought for sure it broke a piston or worse, but after removing the heads I found that there was absolutly no where for any more carbon to form, it was packed solid!
Cleaned it up and its still running for the current owner with over 6000 hours on it!