I have a 16s.21j. that only runs when the crown
is twisted and maintained on the crown. When the
crown has no pressure the watch stops running.
The balance swings free with a good arch. Just
puzzling to me.

Roger my guess would be oil that has dried up over the years and the mainspring probaly set also and can not apply enough power to plow through all the dry oil.

I would also say a lack of mainspring power. The spring itself may need lubrication so the coils can slip and unwind. I had a clock that kept stopping and oiling the spring fixed it. Of course it may also be excessive dirt and therefore friction elsewhere in the movement.

Thanks Sammie and Donald. I guess a COA is needed.

Anytime you have to put pressure on the crown to make a watch run, it's time for a coa ,it's either dirty or dry and binding and cutting power in which will not run, all pivots must be completely free which allows all power from the main to be distributed down the train of gears, and by the time that power gets to the escapement wheel it only takes a spec of dirt or old grease to halt the movement on power.