I know this isnt exactly going to help alot regarding the issues you posted but if you are after going to be interested in a webber upgrade have a chat with recarb..
http://www.recarb.com.au/advanced_sea ... .php?keywords=a15&x=0&y=0
(link is for A15 manifold and carbs but will bolt onto your A12 - but they have brand new single for A12 and others just give them a call)

Here is the link for the A12 single side draught
http://www.recarb.com.au/advanced_sea ... .php?keywords=a12&x=0&y=0

You can pick up a twin Webber set and also manifold from them.. or single carb and manifold

I got my twin 45mm Webbers and redline manifold for my A15 from them.. great guys with some awesome help, they will also tune the carbies to your engine specs if you provide them..

I had mine tuned and sent, bolted on with no issues and runs a treat starts cold no choke most days and run a treat..

I cant really give an exact idea though on the heat displacement from them but after a long while they still are cold enough to touch by hand