Modern Vespa

If you have a look at the video the back wheel does not always turn when using throttle almost has a delay ? When I didnt know much about bikes the owner i bought it from told me thats just how it is but Im thinking its not getting enough air ?

Sometimes it dies after reving really hard ? what could be the cause for this ?https://www.youtube.com/watch?v=A7-WLJ66WJY&list=UUu-6C7Hk-CVfr7sIz_j7pUw

I'm not sure where t begin. The delay is probably caused by tight clutch springs and is a good thing because it puts you in the power band before you move. I would open the CVT and clean it up. Check the condition of the rollers, belt and clutch pads. It may just need to be refreshed. The same goes with the engine dying. New plug, carb cleaning, hoses and filter as a start. Did you upjet the carb when you did the 70cc kit? Otherwise it starts and sounds okay, better than most that ask.

Carb has not been upjetted, it ran fine before, may have to upjet it to see if thats the problem, added new spark plug, changd filters checked the rollers everything seems intact.

Rule of thumb is upjet by 4 going from 50-70cc. I ended up going up 3 from a 68 to a 71.

And as above that delay will be your clutch springs which probably have been replaced for stiffer so the the clutch has to spin faster before engaging the wheel.
If it's pulling away strongly it's probably ok, if it's popping a wheely go lighter, if it's bogging down go stiffer.

Oh and you might need to adjust the needle on the carb, very doubtful it's correct after going up to 70cc.