wats the best carb for the zbox 49cc engine kit

[2fast4u]

hey ok first ill tell u wat is goin on when i am at close to full throttle my engine seems to back off then speed up and at full throttle it feels like it has droped its top speed i use to do 55km/h now it feels like 35km/h im suspecting the carb but i would like some more opinions on the matter so if the carb is the problem can any 1 vouch for a good carb