Urgent help need ideas to try narrow down

All righty guys my house mates Aerox was a bit sad so we have done a full upgrade stage 6 sport pro everything including pipe vari and clutch now it just won't run when you grab the throttle it just back fires and runs like a bucket of Poo! We have tried different jets needles and all so different carbs! From a 17.5 to a 19 and 21-24mm and it still reacts the exact same way every single time it makes no sense to me at all and need some serious help and ideas as to what this problem could be. Second problem is another mate bought a Aerox as well with a eBay junk kit in it same sort of problem but it runs fine at about qtr throttle any more it breaks down in the revs very fast again we have tried jets needles and different size carbs. Please throw any ideas you can at me so we can try get these scoots running properly about 4 weeks we have been at these scoots have all so tried new plugs etc

 

Id be doing compression test + looking for air leaks on the intake on both scoots.

 

^^ Whut he said, plus I'd be checking that the coil and CDI are both OK. Bad timimg or CDI issues can cause backfires.

 

Thanks guys for your help I shall get on it asap