Feroza flare up

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Hi fellas. first time poster and Daihtsu owner. I have obtained a cheap Feroza EL (carb) for my son who like bombing around Dartmoor and puttng things in hedges! Ideal. The car was an import in 1998. I have noticed an anoying little problem, when I lift off to change gear, the engine flares up by around 600rpm and slowly drops back. Is it a vacuum type problem....Any ideas.