Every time that's happened to me it's either been the valve or an air leak somewhere that makes the valve keep operating in an attempt to stop the engine stalling.
Take off the valve. Give it a generous spray with carb cleaner.
If that doesn't work, then have a hunt for an air leak pulling air in through either a gasket or seal.
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….And the latest thing to break is.....
### MG STANDS FOR "MY GREMLIN" !!!
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