Originally Posted by
A and C
The problem was only found in the hangar after the icing incident referred to above when pressure was applied to carb heat valve to simulate airborne operation, as soon as this was done the crack opened up and the defect became apparent.
I have no doubt that the crack quickly propagated between maintenance checks from something so small that detection was very unlikely to the three inch crack right across the width of the valve that caused the problem, it all goes to show that no matter how careful you are aircraft will invent new ways to kill you.
This is a pretty unusual failure mode. Most issues with inadequate carb heat involve a cracked carb air box, broken butterfly valve or a loose/deteriorated scat hose, all of which are pretty obvious.
Getting the carb heat full on should do the job, although many new pilots have been caught out by the fact that often the first thing that happens when you apply full carb heat is
the engine runs worse 
, this is because the engine has to ingest all the water you just melted. Engines don't like even a little bit of water. The temptation is to push the carb neat back to cold, but this must be resisted. The good news is that the engine will pick up in a few seconds and full power will be restored.
However systems knowledge is always a good thing and this discussions is a great example of this. Under normal circumstances full carb heat should clear very quickly, within 10 or 15 seconds of application of the heat.
If the engine is still showing signs of carb ice with the carb heat full on for an extended period, something unlikely but certainly possible, than more measures need to be taken and this is where leaning to increase carb temps and provide a more favorable fuel to air mixture for the iced engine would be a prudent course of action.