X7 Questions

Hey guys,
So i got my new X7 and have a few questions about it. I've owned a pocket bike before but not a super pocket bike so i am not sure what is normal to this machine. Anyways sometimes when i'm riding and i decrease acceleration really fast (to stop at a sign or whatever) there will be a series of popping noises (i don't know if that's backfiring or not) anyways i was wondering if that's a bad sign and how could i fix it. Also another question is when i got the bike idling and i lift up the back wheel, it will spin, not very fast but still. I was also wondering if that's normal or should i adjust the idle screw? And my final question, the oil on the bike gets dirty really fast. The first day i got it i replaced it with castrol 10w40 oil and after about a ride the oil got pretty dirty, i don't know if this is normal since its a brand new engine but please let me know what i should do.