Weird hairspring issue

[Simeon]

I have a bit of a strange one with a balance that only runs when the hairspring is being touched.

Installed the balance but the watch wouldn't run. Flicking the balance with an artist paint brush, the balance swings and the pallet fork moves and advances the escapement. I take the balance back out again, check the draw of the escapement by cycling it, it looks fine, cleanly snapping from side to side when touched.

Reinstall the balance, the balance starts up - I decide it's 'just one of those things'. I put it on the timegrapher, it's a snowstorm and can't get a reading. I look carefully at the hairspring and can see that the outer coil is caught on top of the nearby centre wheel. 'There, that's the problem' - nudge the hairspring off the centre wheel, it pops neatly under the centre wheel and sits with visible clearance. Balance stops.

I look carefully at the hairspring, it looks flat and it is centered and is clear of the centre wheel. I touch the outer coil on the side where the centre wheel is with an old oiler - balance starts again. I take the balance back out again, lay it upside down and it looks fine. I can't quite see underneath the balance cock when installed to see it it is touching (and moving the outer coil away from the centre makes it clear again) but it seems to be centered OK when installed or out upside down. I have had it in and out a couple of times now, I have also caught the outer coil on the centre wheel again and it ran (badly) again.

What could this be? I am going to take the balance off the balance cock again and see if there's any obvious issues but I can't imagine what they might be because it looks fine installed. 

[nevenbekriev]

Sounds like the hairspring is pressed on the balance spokes. Take good look from aside, and under the cock too.