On a stock framed XS650, with any linked carbs, like the stock BS 34 and 38, Mikuni RS34, Keihin CR/FCR, and the popular Kawasaki 500 flatslide CV carbs, there is no room to move the carbs further back. So any lengthening of the intake tract will be by adding velocity stacks or straight runners between carbs and pod filters. And those will be roughly 55-60 mm in inside diameter.
With separate carbs, like Mikuni VM or TM, various Keihin and clones, and DellOrto, it is also possible to have longer intake tubes between carbs and head, typically with 34 to 36 mm diameter, like these two options:
https://www.heidentuning.com/racing-parts/carburator-manifolds-racing-aluminiun-set-detail.html
https://www.heidentuning.com/racing-parts/carburator-manifolds-racing-aluminiun-set-1-detail.html
Or use velocity stacks between carbs and filters, like on linked carbs.
Now here is the question: Given the smaller diameter when adding length between carbs and head, this will also give a higher gas velocity, and hence more inertia for the incoming charge. Will this be more beneficial to power than adding a velocity stack behind the carbs? 90s carburetted Ducatis have 6-7" long intake run ers between carbs and heads, and only an approx. 1" long rubber adapter/ bellmouth between carb and air box, as shown in drawing on top. (Dunno why it ended up there.....)
Which is very close to that on the Ron Woods Norton, pic. attached below:
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