You seem to have done everything right.  Are you suffering from low humidity?

Please describe your instrument more.  What kind of jacks does it
have?  Are the offending ones sufficiently loose in their guides?

It is important to remember to disengage the damper completely when looking
for causes of hanging.

It seems to me that the "ridiculous lengthening of the jackpin" you
describe would make the plectrum closer to the string, and thus more likely
to hang.  Hence the suggestion that the jack (if tapered) is too good a
fit, or bent.  Have you tried moving jacks around to see if the adjacent
jack works in the register, and vice-versa?

Are you sure that it is the jack which is not descending, rather than the key?

Forgive me for stating the obvious.

david

At 12:06 23-01-01 +0000, you wrote:
>I've been really getting to grips with harpsichord maintenance.  My
>instrument has been a real pain in the neck in its first nine months,
>but Polyanna-like I can say this has at least had the advantage of
>forcing me to learn the ins and outs of repair quickly. Many thanks
>to the list for tacit and active encouragement and help. I now have it
>regulated--a happy state it has only achieved for a few days up to
>now.  This made life so much easier I began to suspect some of
>my deficiencies in playing weren't due solely to bad technique. I
>discovered several keys no longer obviously sticking, but hindered
>enough to make trills difficult, and several slow hangers.  Most
>have responded, but there are still about 3 that have improved only
>to the extent of hanging one time out of ten instead of one time out
>of five. (If it likes the weather one time out of twenty.) The worst
>offender finally gave in to a ridiculous lengthening of the jack pin: of
>course now the damper leaks and the note doesn't stagger. I've
>tried: changing or cleaning plectra, adjusting springs, restricting
>tongue movement with a thread around it, adjusting height and
>angle of dampers, and last resort, usually cautious change of jack
>height.  This routine worked for most of the offenders.  Is there
>anything else to do?  A little extra weight on the jack? Sacrifice a
>black chicken?
>I also have one key in particular that comes down with a thunk all
>too often. So I suspect the problem is partly mechanical.  The
>keyboard is very simple: pin in front, pin in back, whole thing
>comes out like a tray. Is it just me or is an adjustment possible?
>Thanks,
>Jenny