I Installed a Dellorto VHSB 34 LD on my 2009 125 Halley to replace the VHST 26 that was in it. I got the carb used, pretty cheap off of an Aprilia RX 125. The bike won't rev above 7000rpm when in gear (the tach is a little optimistic in my opinion so it's probably lower). I replaced the 132 main that was in there with a 125 and not a whole lot improved. At one point I removed the filter to let additional air in which introduced a lean WOT bog at low revs, it still wouldn't rev out above 7000-8000rpm. I cleaned the carb before I put it in. The bike picks up quite nicely, then just as it's about to enter the power band it starts sputtering. The strange thing is, that it used to sputter (albeit a little higher, at around 8k) with the 26mm carb too. At first I thought it could be the timing, but the manual for the bike says that it should be revving to 11K stock, so I guess the problem lies somewhere else. My guess would be the float level, my dad eyeballed it since I didn't find any info on how high it needs to be. I now know that it should be at about 14mm measuring from the float to the gasket base. I did a little road test and I recorded it. I hope it helps.
https://www.youtube.com/watch?v=JG7e...ature=youtu.be
PS: When I pulled the plug it had a pretty nice color, it was mostly brown with some black (a little rich).
Update: Adjusting the float and removing the acoustic/backfire baffle in the air filter frame changed the situation for the better. Using a 145 main jet it now revs all the way up to 9000, it starts pulling at around 6500 and the pull becomes a lot weaker at around 7500, until it stops accelerating at 9000-9500 RPM. Washing and re-oiling the filter did not help.