XS650s came with both 38 and 34mm CV carbs as standard. Parts outlets sell kits consisting of 34mm carbs, either flat or round slides, manifolds, boots and so on. Single carb fitments are usually 34 or 36 and single carb manifolds are available.
There are various carbs available, Mikuni, Delorto, Kehin and a few knockoffs, but any carb would work provided the venturi size and jetting are correct. Best to stick with a known rather than an unknown though. Mikuni would make the most sense as parts are readily available.
I have a pair of 34mm VM round slides on homemade manifolds, they work fine.
Commercially available kits come with two part manifolds which site the carbs further back than normal, siting the carbs directly under the petcocks, so you either have to use extended manifolds as I have, or tilt the carbs over as most seem to do.
VM34 carbs have an outer spigot diameter of 40mm. There are 42mm manifolds available from Hoos which some people have used, thus siting the carbs forward of the petcocks.
Kits make the most sense as they are jetted to suit the XS650 and jetting info is readily available.
