Ribbons are not the OP's original problem. There is a delay in
exectution of tab, space, backspace etc.

I would guess that it needs a belt. I have my reasons. I had a
Selectric when I was younger, so of course I eventually took it apart.
Mechanical engergy is moved in a slightly different way when it does
not have to move the typehead. The three functions mentioned are
exactly that.

The reason might be that those three functions are induced directly by
the keyboard, any actual typing the mechanism assures that the
carriage does not move during the strike cycle, no matter how fast you
type.

The best way to insure this is to have the typehead driver mechanism
invoke the carriage move after the strike cycle.

You might look in there and find a rubber belt dried up. Possibly
rejuvinate it. Or if it even has a leather or cloth belt (I would not
be surprised) there might be a way provided to tighten it.

I was always fascinated by Selectrics, I never did learn completely
how they work, like the specifics, but their theory of operation is no
mystery to me.

Yup, if the keys work well and the space and stuff does not, look for
a belt. Hey if it's leather or cloth you can just wash it and throw it
in the dryer if no way is provided to tighten it. After doing so
though, measure it, it becomes more likely to break.

JURB