Also, when you say it is running out of steam, is it doing it at the same point regardless of margin settings, or is it getting stuck around the point it reaches the bell?

If you haven't already, move the margin setting to the extreme end to check to make sure the slow down isn't related to the movement being caught up in something related to the bell or other part of the margin mechanism. If it slows down regardless, I would then check to make sure there isn't a bind by holding down the carriage release and moving the carriage back and forth trying to feel a bind.

If you have eliminated possible binds, and everything is clean, then you can move onto mainspring tension, though I usually measure the tension first, attaching the end of the drawband to a digital luggage scale, before messing with it. I cannot remember the recommended tension on Royal portables, but for their standards, it was 1.5 to 1.75 pounds--so it would be less than that. If your tension shows that it is already above that, than your issue may be something else and you would just be compensating by increasing the tension and not solving the underlying issue.

I happened to have torn down and rebuilt my '47 QDL this weekend. Haven't got the body panels back on yet, so it's easy to look around. 

One thing I'd suggest: strip the carriage down to essentials (i.e. pull out the platen, all the rollers, and the paper tray) so that you can see through the carriage. I found it really helped me on my 40s vintage QDLs understand what was going on and how the carriage worked. 

Also, Lucas Dul's video on installing the carriage is good for seeing what tall the moving parts are and how they fit together. (Though he starts the video with a model P, he switches to a QDL around 1:30). His skeleton carriages gave me the idea for stripping mine down for debugging. Type Shop, Ep. 2: HOW TO INSTALL A CARRIAGE ON A ROYAL TYPEWRITER - YouTube

On my vintage machine, the carriage rails sit on 4 ball bearings, each with a little orbital gear. Those plus the pinion gear that meshed with the carriage rack were the only points of contact with the base. I could imagine that those bearings being gummed up could slow things down. I took my carriage completely off, which I think is overkill. I wouldn't recommend it except as a last resort as it's pretty hard to get back on with the bearings in exactly the same place (impossible without Lucas' video, above). When I go after my '41 QDL next, I'm going to leave the carriage on, but strip the rollers and paper path out so I can go through the carriage to access stuff below. Then perhaps spray solvent through a straw down the rails to clear out any gunk. 

If you do take off the carriage, I recommend putting the typewriter in an old cookie sheet with sides before you start - that way when the bearings and gears fall out, they won't go anywhere. (I always use a cookie sheet when working on typewriters, carburators, or anything with small parts for that reason).

Big caveat: I'm still a beginner in the art of typewriter repair. Take everything I've written with a big grain of salt. 

overwood wrote:

the carriage only runs out of steam past 70 on the paper scale.

Is this pica or elite? I am trying to imagine how far down the hangup starts.

SoucekFan wrote:

Is this pica or elite? I am trying to imagine how far down the hangup starts.

Pica.

SoucekFan wrote:

Oh. I just remembered. This is probably not it, but easy to check and rule out: I once had a Royal portable hang up like this, and it turned out that the cause was the knot in the drawband on the right side of the carriage was rubbing against something when the carriage was nearing the end of the line, either because of the way it was hanging or because it was a hair too big, and it was causing a slight bind.

No I don't think its that, I checked that when I had the platen out. It feels like either the bearings or the rails, Got some sewing machine oil and will try to clean the rails again (hard to clean what you can't see) before putting a small drop on it.

Keeping fingers crossed...it seems to be working OK at the moment, after 2 rail cleanings/lubes and with maximum tension on the adjustment knob...hope its not temporary.