Submerge them in petrol?

I would have thought a thin smear of red rubber grease (Castrol 5900) might be suitable, it did wonders for my shrunken diaphragms in my carbs :-)

I will get some of that stuff.
I didn't put enough in my first post, though. I should've said that I'll be doing the valve clearances, re-jetting etc, so I'm thinking in terms of having them out of the bike for several weeks. They may just perish regardless of what I do.

Several weeks?! Are you hand crafting the parts with your feet?! Lol only kidding mate, they should be ok, just bung em in a pot of petrol for a couple of hours before you fit them to soften them up, they should be fine

Am I missing some thing?

Why would carb rubbers perish when taken off the bike, just sat on the bench?

I see no difference to leaving the bike sat for weeks with them fitted.
They go hard over the years but can be softened with heat for refitting

They can stay on the heads anyway, just remove the carbs and grease the inside of the rubber to refit them.

Fair do's, if you guys say they'll be ok then that's good enough for me!

Sam, I'm probably gona SORN it and take my time with valve clearances, etc, so I'm in no rush at the moment. :)

you on about the carb manifold rubber s? when i had my carbs off for a while i ties some plastic food bags over then with some 4t oil smeard inside and on them, tied it off with some hair toggle jobbies and lasted for like a month and half, still supple as a thai hores legs.

soak them in some hot water before re-fitting, smear a little oil over the lips, and the carbs snap back on with the lightest of pressure, as opposed to wrestling the whole bike trying to get enough leverage with old, hard carb rubbers...