On jet size and needle positions...
Tuning this particular carb for this particular 2hp has been trickier than I expected. I've identified the better needle settings, tested all the jets, and found all the sweet spots for the air screw and idle screw for the different main jets... But this Nibbi has still been more difficult to read than your usual Keihin or Mikuni. I am getting closer, but the settings have gotten weird...
With eight main jets from 108 thru 125, I'd had high hopes for the largest four, but while they might have improved certain upper power bands, they would also include multiple other bands of sputtering. The bigger jets were just too rich. Strangely enough, I found that if I also richened the needle jet by raising the needle position those sputtery bands started to fill in. This allowed me to venture back to the smaller main jets while keeping the needle in position #5. So now I'm now back down at the second from smallest main jet (#110) and road testing pretty well, so long as the needle stays in its highest position (#5). NOTE: I've even raised the needle to extremes, with as many as 4 shim washers, and with success... So go figure.
As a final approach, I've ordered one smaller main jet (the 106) and two larger slow jets (42 and 45). The #40 works here, but I want to see if I can fill in the lower band more, and maybe even bring the needle back down a bit. I've also resolved not to adjust any higher than a "realistic" needle setting (like 5, plus one shim). While raising the needle seems to smooth out all the bands so beautifully, the sky can't be the limit, and Nibbi doesn't sell different needles...which I also think is curious.
This bike is unique, in that it has a little 2hp engine and huge 72 tooth sprocket. It's a bike with little torque but fun top speeds. In other words, there's no reason not to floor it for much of the ride. And I'd heard racing carbs (like flatsides) are designed for this all-or-nothing fuel delivery, so I had hoped this Nibbi "Racing" carb might be a good fit (plus the colors were a good match). And in fact, the Nibbi's slide throttle valve is so wide, and the slide travel is so short, that it kind of operates like a frickin barn door for fuel. So I'm into it, and I'm not cutting my losses just yet.
But if I can't smooth out the whole throttle range with one of the 9 main jets in my extensive inventory, and one of my 4 pilot jets, and a needle shimmed above #5... well then I'll just switch carbs. Stay tuned...
https://flic.kr/p/2nMRsHn