Scooter Problems

[dirty ern]

Hey Guys, I just bought a Vino 125, and it came with a box of spare parts. It appears the previous owner put an aftermarket variator and sliders. Also has a custom air filter. The scooter is much faster than stock however there are quite a few problems. The main issue I need to resolve is that when I apply throttle from a stop, there is a skipping, the bike doesn't move but it feels like the back of bike is thumping. then it takes off and accelerates normally. Any help would be great!