Unscrew the nuts holding the carburettor to the inlet manifold studs. Lift it off, keeping it right way up as it contains fuel. Tip this away safely. Leave the gasket on the inlet manifold as a guide so that you can fit the new gasket the right way round. Keep the carburettor upright when lifting off.
Over a period of time, dirt inevitably gets through the fuel filter of a carburettor, and chemic...