Day & night Variations in 16013 speed?

[GGGMT]

My 16013 averages about +4 - 6 / day, which is fine. However, since timing the watch closely over 4 days, I note it runs very fast during the day +20 and very slow at night (ie right when I wake up it’s running slow).

Is this normal for this period watch motor (1987)? Atypical?

Thanks very much for any insights!

[tamiya]

Is it fully wound?

When was it last serviced? If over 10yrs... main Spring could be getting tired, at less than fully wound it might not be supplying adequate tension to the drivetrain. Aged oils will have dried/migrated and friction is slowing the works etc etc

[GGGMT]

Originally Posted by tamiya

Is it fully wound?

Thku for the reply. Yes, fully wound (in other words I’m wearing it full time. Last service ~2-3 years ago.