This is generally always caused by a carburetor issue..
did you play with any of the screws on the carb? you are probably running way too rich or way too lean if i were to guess.
or maybe you mixed the fuel wrong if it requires pre-mixing.
or it could be a coil or spark plug issue, or the timing... those are all things to check. start with the carb though...
take the carb off, clean out the bowl, make sure the jets aren't plugged etc...