The new GXII Gen6 carb is a three needle design. It's not actually a needle, but a mid range fuel distribution tube for want of a better description. Very similar to the OS 3 needle carbs.
The fuel tube allows for a better delivery of fuel across the rev range, plus the new carb has a slightly wider diameter throat for more effecient operation. It seems to allow you to run the HSN somewhat leaner than the original 2 needle designs without getting the lean bog, so more power and more economy.
The combination of the GXII crank and Gen6 carb brings the bottom end of the MG66 to life more, while still retaining the insane but smooth mid range and top end this brilliant motor has. Seems to also be a little more power right across the board (as if you needed it ), which is probably due to the more efficient carb.
Food for thought once you are all bedded in and going hard .