The standard jets on the factory carbys are way to big IMHO usually .68 ,my suggestion is to go down to a .65 and see if that helps
But we really need more info such as jet size the oil your running and the type of carby ( I suspect the NT )
There can be only two main suspects in my oppinion and they are jet size and fuel mix .
Being a newbie you can get somewhat daunted by the carby adjust but it's simple ,I always run in my motors at 25:1 and general running after breakin at 32:1 ,you should find if you are running the kit recommended mix ,swapping down to 25:1 should somewhat fix your four stroking issue ,40:1 for breakin is IMHO way way to low to be any good for the motor
Try these and get back to us with your results
Regards Hen