Thanks again Will, now you really have me scratching my head
and wondering if it's not all my imagination or ears. I won't be down to the boat until January but when I do I'll pay closer attention and double check the sync against a test tach. Interestingly Glendenning suggests setting the lead engine
25rpm ahead of the slave. This of course means that whenever the Glendenning is decoupled, there will always be a discrepancy between tachs so maybe mine was purposefully set 100rpm ahead rather than the 25 suggested.
Thanks for the link Sailmonkey, yes I did find that.