I got intrigued and tried this. Got an old-ish set of carbs off e-bay for $35 and set out to rebuild them. They were seized, throttle shaft and slides. It took some patience, pine sol and carb cleaner but finally it's all squeaky clean. Should have taken pics this was epic. Luckily all ended up intact: diaphragms, slides, throttle shaft etc. Cleaned up surprisingly nice and only stripped 2 screw heads (GD JIS screws).
Then I got the recommended 38/138 jets and JJJ needles. Very easy to adapt the needles thanks to mrriggs.
I thought the cleaning was decent enough until I hooked up the carbs and gas poured out the vent tube. Bad float valves. I have spare Hawk GT carbs and they use Keihin carbs with the same valves, problem solved.
The throttle cable setup is a bit ghetto but it works decent enough to test. All I did was clamp a bit of rubber in a shaft collar into the bracket.
Fire up and after adjusting a bit so that it would run on both cylinders, it's quite the shocker how well this works. I still have some off-idle issues and hanging throttle, I suspect pilot jet/screw and needles will need tweaking. It does idle like a swiss watch, and takes off like a raped ape. Also intake noise is different, coming from BS38 and pods (I actually use the same UNI pods, they fit well), hard to describe but it's less of a 'bwoop' and more of a solid 'braap' noise. Better to my ears in other words.
No doubt this is a worthwhile mod., very impressive. The motor gets this modern bike feel when running, just when I thought my BS38s were dialed right. Also as was pointed out they probably weigh half what the BS38 do. I'll weigh them when I get a chance.
Thanks guys, appreciated.