Peugeot Speedfight 50cc AC two-stroke oil in airbox

As the title suggests, after a longish run (5+ miles) there is what looks like a mixture of petrol and oil on the transmission casing. When I check the airbox it is full of this fluid.
This fluid is presumably leaking back out of the carb, can anyone suggest why it is doing this? I expect it is happening at WOT (which the scoot is at most of the time!)
Secondary symptom (may or not not be related) is that it only really runs right at idle or WOT, it is very boggy in between. It has a new carb (scooter specific) purchased from WEMoto if I recall so should be the right one for this scooter. I have drained the float bowl and the fuel is E5 in the tank.
Any suggestions gratefully received, many thanks in advance!

 

Have a look at the reeds to see if there warn atall.

 

Thanks Marsh, I'll take a look at the weekend

 

New carb will want jetting to the bike, not all jets are the right ones even though they will start and run it

 

Thanks Steve, I will check the jets