Sorry if you've already seen me post the same subject on PCGB, but I thought some learned opinions might also be available here. Here goes:
First run out on Sunday after numerous works to improve the car. I was meeting the PCGB R19 chaps for the run up to the Cornbury House Open Day, pulled up after an uneventful commute to the meeting place, and got out for a chat while everyone assembled.
On starting up, the engine was mis-firing, but to my horror, great clouds of black/blue smoke were pouring from the exhaust. The engine settled down to a normal idle after maybe 30 seconds, but the smoke persisted for a while. I set off anyway, and the car performed normally throughout the journey, during which it received no quarter with an Italian tune-up. Another S2 was following me on the journey and reported no smoke once the initial cloud cleared- nothing on change up either.
It repeated the smoke problem when I started up to leave Cornbury, but not to the same extent as earlier. I've parked her up,

ed...
Any ideas chaps? I'm thinking sticky valve, broken valve spring, something like that.
Thanks, Craig