5Twins, you may remember a few months back when I had a similar problem where my bike wouldn't rev up or go over 60. I tried tapping the right carb on the side of the bowl with a screw driver handle, while riding, and it cleared up. I suspected that one of the float needles were sticking. During my rebuild I kept the carbs upright and took them apart. No sediment, didn't expect it since I always have those little funnel shaped filters on it.
Well, I replaced the float needles and seats, an inexpensive kit from Mikesxs, and now, after rebuild, no problems.
Hey, I also replaced the float needles and seats when I put it on the road again in 2014 and it ran great for about 20K miles.
With all the vibration that goes on, those little needles, whilst hanging on those little wire holders, I'm sure get knocked around a lot creating micro nicks and dents.
Also, upon removing the head, the intake port and valve on that right side were very wet with gas.
Yes, definitely, 12.7V is not enough.