Spark plugs.

ok, that would explain it smoking a lot on the left after start up after it has been standing a few weeks on the side stand...

Valve seals... that exactly what I was getting at. If we can get a look at the plugs the fouling should indicate if it oil or fuel that's the problem.

You can also shine a light into the spark plug hole and rotate the engine so you can look at an open valve, if the back for the valve head and stem are oily and heavily fouled that means your seals have been leaking for a while.

Iridium spark plugs is a must do for Z1300, since using them I have noticed a good improvement in starting and an overall stronger cleaner combustion performance, better mpg less throttle required. If you look closely at the earth electrode there is a nice tiny circle burn from the spark which shows a more accurate ignition process than standard plugs. Obviously the Carb's need to be synchronized and mixture to be set correctly to feel the results. I use Gunsons colour tune to set mixture/combustion to a sky blue colour. Ignition timing, clean air filter, to set good combustion colour. It's all in the detail.

I am a bit weary of ngk plugs like this since the platnum tip plugs that the tips dissapeared on.
Has anybody lost a tip from the iridium series.