these things can realy seize on - hot water, then an electric paint stripper (less likely to melt things) to try and expand the 'stack'.
Often better to use an ajustable spanner, open just enough to fit the lip of the 'sack', then used to twist the stack off, is better than any clamping arrangemnet, or pipe grips
failing the above, you will have to cut it off - again a Dremel & cuting disc, is an ideal device.
had to remove a couple of those short 'bell-mouths' lately and they are swines to get off in one pce.
This should be done with the carb OFF the engine for obvious reasons! Squirt some methylated spirit over the bell mouth and set it alight! This heats it up quickly but meths has a fairly cool flame so will not melt anything. Then try and unscrew by hand using something grippy round the bellmouth, such as rubber matting. Repeat as necessary. Usually heat will do it. Other way us with an electric heat gun, indeed, but the meths thing is quick. Try screwing ON as well as off.
