Anything and everything Spitfire.

I have a mark 3 spitfire which has been misbehaving for a while, I knew that the mixture was wrong based on the colour of the spark plugs so though that this might be a good place to start.

Following the haynes manual instructions I have;
checked that the pistons fall correctly,
screwed the jets up flush to the bridge
backed them down 2 full turns
put the dashpots/dampers back on
started up, adjusted the idle speed and balanced the carburettors
fiddled with the mixture nuts using the engine reaction to the lifting pins as a guide

All seems well until I come to stop the engine at this point everything seems to go wrong. The reaction includes some or all of
the sound of an explosion in the exhaust system somewhere
the engine spinning on and firing great plumes of smoke out of the carburettor intake and
what appears to be raw fuel being ejected from the rear most carb (from a small opening just under the fuel line.

Can anyone suggest what I have done wrong as I am really struggling for ideas

Cheers

Paul H