I have a JVC cassette deck from about 6 years ago that is one of their
top-of-the-line models -- auto reversing, dual deck, and when it records, it
tests the tape by recording a test signal in order to adjust its bias. Very
nice.

Its one flaw is this: It's not willing to apply much torque to the tape.
If it encounters a tape that has minor mechanical problems, it will flutter
badly or auto-stop when a cheaper deck would have just kept it going.

Being gentle with the tape is probably meant to be a virtue, but I'm needing
to copy a large collection of older tapes in very variable condition.

Is there likely to be an adjustment inside to increase the amount of torque
that it can apply?

It has always been like this, which is why I don't think it's a worn-out
belt, although it's conceivable that it left the factory with a belt too
large or greasy. I've never opened it up.

Thanks!