try this;

http://www.nsr250.net/forums/viewtopic.php?t=3959

'18 barrels fit on a '16 block, they have bigger exhaust ports and need '18 pipes to help the gases escape. '16 carbs are smaller than '18,'21 and '28 models and will fit on a '16 with some mods. Once all this is done it should make 55-60 bhp. I'm starting to collect all the bits required also
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it's a name, not a condition