Thanks for the replies.
Pete, what I meant to convey is that the motor is now to standard M21 600 specs.
It has an M20 carby (276 type) pending fixing a proper M21 size. The needle's raised a notch. While not correct, it's run another 600 well for years.
I'll try Ian's suggestions if (when) it happens again.
I said that patches on the bore were rust coloured, thinking about it they're better described as amber & I wonder if the bore's glazed in these areas & whether this would have anything to do with my problems.
An M20 carb is 1/16" smaller than an M21 carb (1 1/16")...That in itself wouldn't matter, other than it's a slight restriction, as long as it's set up OK...
The M21 also uses a different mainjet...That's smaller than an M20 at 160 instead of 170, so if anything the M20 jet would be a little rich (assuming the carb isn't worn out)...However, a slide that rattles about in the body could negate most of the above statements and the carb may well have to run with non standard settings to achieve the correct mixture...The other parts that wear over time and can affect the settings are the needle and needle jet...
I wouldn't imagine bore glazing, if you have any, would come into play with the problem you describe...Ian