i had the exact same problem when i first got my hyper .. check where your bottom end is .. richen both bottom and hsn by 1/4 turn .. its the carb thats doing it.. hypers have dodgy carbs..

If that dosent work open up the HSN by turning it all the way out and see if there is an obstruction.. like a piece of rubber or something .. also when you have the HSN out check that you have a clear passage from the recieving collar to the fuel jet..

failing this you can buy a new carb .. OS 20 e , any RB or Novarossi carb will do it.. In fact any 21 up to 26 carb that fits will do it.. (preferance in that order)

Good Luck