I was testing my 3570 Speedmaster 'moon watch', it runs for about 49 1/2 hours from full wind. At around 46 hours its accuracy decreases dramatically, like loosing >20 seconds an hour. My watch is 4 years old, I am wondering any members here experienced this. Can this be a main spring problem showing its age?
Sounds perfectly normal, from full wind, to 24 hours down, to almost unwound the mainspring power output varies, and with it the amplitude and timekeeping of the balance. A manual wind should be wound off the wrist at the beginning of each day, that way it has the best chance of keeping good time all day. The reason that autos keep such excellent time, is that their mainsprings while worn are at or close to full wind all the time, giving the balance a good strong amplitude and accurate timekeeping. In your example as the amplitude decreases the hairspring is wound and unwound a smaller amount, and as the amount decreases the hairspring doesn't interact with the regulator pins as much or at all, which in effect lengthens the hairspring, a longer spring vibrates at a lower rate, which matches with your findings.
Getting 49.5 hours is great. At the very very end the time keeping can be more erratic as it barely has any power left.
Why should it be wound "off the wrist"? Sorry if dumb question but just bought a new Speedy after 20-odd years of automatic watch wearing so am keen to do things correctly.
It's recommended to wind off the wrist so that there isn't any side load on the stem or the keyless works. It's your watch though and you can wind it anyway you like, I am only offering my input as an independent watchmaker and what is recommended as a general practice for handwind models.
I appreciate the help in your replies. This experience was different to automatic Rolex watches, they seen to be accurate until the last drop. I find winding off wrist to be more convenient.