Check your carb block that hols the carb to the motor. If it's loose it wil leak air in and you will lose the vaccum to the walbro carb so the pump wont work well. Once it warms up the carb block exspands due to the heat of the motor, helping seal off the leak.[:-]
This could be your problem due to the motor being new the fasteners tend to loosen up after a short time of running, also check your plug for fouling, if it's fouled it will run rough for a short time until the clyinder tep reaches the upper limit where the mixture ingnites easier. Just check out the mechanical conditions first before changing any of the mixtures.