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2000 falco, has sat unloved for the last year. Started and revs surging on throttle between 1000 & 5000 revs when warm. If you hold the throttle the revs will fluctuate about 40%. After a short run and fresh petrol seemed to improve and ran well, 24 miles later starts surging again. A few pops from un-burnt fuel when it kicks back in. Feels like its going onto one cylinder but both pipes seem about the same temperature. Makes for entertaining riding in the rain! Have checked diagnostics and no fault shown. Had a quick look at the wiring and can't see any issues. Will tick-over ok thought slightly slower than normal when in a surge episode. Any ideas?
Was planning next to change all the vacuum lines, any ideas on the range of internal diameters I'll need?

I was thinking along the air leak theme also.
A look at Yorks Aprilia parts diagram may give you the sizes

Check the screws holding the throttle body butterflies on their shafts haven't come loose. If so apply some loctite and tighten them up.