Hi, any help/suggestions appreciated...

Changed the reg/rectifier the other week and bike started up no issue. new battery installed - starts first time....

New issue is around town no problem but when i get up to a sustained 40mph or so, after a few minutes, the revs drop, falters and stalls. Takes a good minute to start up again then same as before - fine at low speeds til hit 40-ish

Any advice/suggestions appreciated. Thanks

knackered vacuum and/or dodgy tap - check you have the proper hose installed on the tap and the cylinder spigot. Is it not getting enough fuel?