Your bike is grossly under-geared. 3% is noticeable, 5% is annoying....you're 12% under, that's huge, though it might not seem so it, in text. However, this motor works best geared for 7.4mph/1000rpm...your setup works out to 6.5mph/1000rpm, in 4th gear. 8000rpm is only giving you 52mph...you should be able to sustain that speed, until the tank runs dry...at 7000rpm. These motors can easily pull low 60s, on the flat, and push 65mph under ideal conditions. Yours is gonna run out of breath and revs right around 58/59mph.
Okay, gearing by the numbers:
17/31 = 1.8235, the optimal road gearing with a 56.5" tire.
53.5/56.5 = .9469
1.8235 x .9469 = 1.7266, the sprocket ratio needed to match the desired revs per mile with your 53.5" tire.
18 x 1.7266 = 31.0788
17 x 1.7266 = 29.3522
Closest sprocket combos are 18/31 and 17/29. Those are edge city, imho. 18 & 29t sprockets are both hard to find...and...involve clearance issues. At the 30t, there's a good chance that the dust shield will get partially chewed by the chain. At 29t, I dunno how much of the plastic will survive. Chain-to-hub clearance will also be marginal. (28t requires machining a clearance shoulder). 18t will fit the Nice, but it's tight; you'll need to remove a section from the C/S cover/chainguard. No worries there; it doesn't hurt anything and you'll know how much to remove...when the noise ceases.
With the carb tuning, you've missed the fundamental point. This is trial & error testing...change something, go for a test run, note the results...repeat as necessary until you get the best results. The process is known, by engineers, as "bracketing"; that means finding the lean and rich limits. Once those are known, the overall range is thus "bracketed" between the two extremes. Since the `80s, air:fuel ratios have been used, directly. From idle through ~1/4 throttle, sustained cruising, the ideal is 14.7:1. Sustained high-speed cruising and at greater throttle openings, the mixture should be gradually richened into the 13s. At WOT, 12.5-12.8:1 is needed to keep temps under control. Getting a migraine? Relax, this is where things become simple & straightforward.
You don't need to find the lean limit; the engine will fall on its face. The rich limit is clear...the engine will act as though the valves are floating. And, dropping 1-2 steps below the rich limit, the point at which the engine will rev-out cleanly, will typically produce the desired 12.5-12.8:1 A:F ratio at WOT. At that point, you're not going to burn parts; so the rest comes down to smoothing-out the midrange, via main jet size/jet needle height combination. IOW, finding the rich limit serves as the poor man's exhaust gas analyzer, in realtime...and it's remarkably accurate.
What you're going to do is a series of test runs. Start with, say, your #150 main and the jet needle in the richest position. Take the bike for a test run, WOT. If the engine revs-out cleanly, go up one jet size and repeat the process. If it goes into rich misfire, then change the jet needle clip height instead. You can either try one groove at a time...or raising it to the middle...or to the top. Working one groove at a time is more precise, from the get-go, but more time-consuming. If you choose to start out with bigger changes, then you'll have to work in the reverse direction, once you find the rich limit. The fly in the proverbial ointment is that whatever main you start off with may be either too big or too small; hence, the bigger initial steps can point you in the right direction a little quicker.
Jet needle position does affect WOT mixture with this carburetor. There is a slight possibility that you'll end up having to choose between two jet sizes...the bigger one with a lean jet needle setting or the smaller with a rich jet needle setting. Start with the basics...finding the biggest main jet that will allow the engine to rev-out cleanly, then working in single steps leaner. Once the rich limit is found you'll be within one jet size and 1-2 needle height settings, of ideal.
