Frozen carb advice

[Stephen Hutchings <s.hutchings@rogers.com>]

Hi folks,
I'm helping my uncle with his restoration of an MG TD, by taking care 
of some of the bits that are common with Healeys. The carbs are very 
similar to the H4s.
I wonder if anyone has a trick to free up a butterfly that has frozen 
shut to the aluminum body. I've got it (and the throttle shaft) 
soaking in penetrant, but I'm not absolutely sure whether it's the 
butterfly itself, or the shaft that is frozen, and it's not really 
something that you can hit!
Thanks,
Stephen, BJ8