Feroza flare up


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.