SXF250 07 dying

Hi, I already posted a thread about clutch drag on a 07 250sxf. Now, I have another topic that I would talk about and I think making a summary here about it will be a perfect solution for everyone who runs into the same issue as it's not popular around the Net. Shortly, my 250f has been rebuilt 30hrs ago having its valves shimmed, a new piston, a new cylinder and a timing chain. Works like a mechanical watch. Starts first kick cold and most of the kicks hot. My issue is that no matter how warm it is, if I give it just a LITTLE bit of gas very quickly and release it, it just revs up like 300rpm but when I do it a couple of times, the bike dies after I release the throttle and then starts in like 7-8 kicks.... I wonder what it could be. Since I bought it, it seems to run a bit rich because if I use the choke for more than 15 secs after starting cold, it dies so I have to push it back immediately after I start the bike bone cold which is weird in my eyes. It also makes a lot of black carbon buildup. I changed the spark plug to an Iridium NGK so the electric part is flawless. Otherwise, great power and great throttle response, feels normal. So that's the issue. Could it maybe be running rich and slowly flooding the engine when doing those throttle blips like a 2 stroke builds up oil that has to be burnt by revving? Also, the bike rarely dies when riding it hot(doesn't happen for me but for my dad so I don't know the exact details) and then again doesn't start in several kicks. In those moments, I use the hot start lever and try to find top dead center to maximise its chance of starting. Once it starts, no big deal(sometimes works off the 2nd kick) and still has a strong punch. It's all weird IMO and I can't quite figure it out... Thanks in advance!