Look Like We Had Some of The Same Ideas
Regarding the emptor and fill tube changes.
Regarding the Adjust-O-Rite settings you said,
"If I let go of the handle the machine will slowly move across the carpet by itself."
That more or less means to me that the Royal is NOT sealed to the carpet. Otherwise, wouldn't it just sit there (kinda like a plunger?) The Adjust-O-Rite plunger is set to "pop" a some preset level, but we don't know what that is, right? It could be satisfied with 60 CFM pressure or 80 CFM pressure, we dont' know...
The two vacs I've tested so far that "self-propel" without a power drive have dramatic CFM losses at the nozzle. Look at my "airflow losses" posts for the Panasonic MC-UG589 and Simplicity Symmetry.
If you want to see a true test of deep cleaning, get a patch of medium pile carpet and put a small amount of flour under it. Then in just two complete passes, look at what's left under the carpet. If the flour is mostly gone, you have a machine that has a good seal and great CFM.
You know, now that I've mentioned that "under the carpet" test, I think I might test all of my machines and post the results when time permits...
Bill