It's a 32mm oval bore cranked slide mikuni. Superficially the same as a SP370's carb though it's lacking that carb's additional primary fuel jet at the base of the venturi. Neither have a pumper circuit. I don't know how they flow as compared to a round bore venturi carb, but it's probably significant that the DR370/400 use much the same notional sizes as the XT500 with similar piston sizes. Although these carbs look as if they interchange it's far from simple, they are very model specific. For instance the 370 carb works happily over a wide range of tuning options, including an altered airbox. the DR400 carb [in my experience] will not tolerate the airbox being derestricted. The obvious differences, bigger body, 1mm extra bore size and only a single taper needle. That last is the killer, and the SP needle is a suzuki unavailable part, not a mikuni standard size. The USA spec DR500 used a slightly bigger carb of I think the same design, 34 or 35mm. Guys tuning XT500s tend to use a round slide aftermarket mikuni for their lack of complication and ease of tuning, rather than any possible additional power. Going bigger may make sense in road racing where peak power is important but it's not always the right way to go in offroad where a bigger carb is likely to dig holes in your midrange.
