rebuilt this bike back last year, and it has been great til now.
the stock carb began leaking, from the primer and from the overflow, and turns out it needed a new float seat.
so, i just bought a new carb.
installed the carb, and when my son drove it off, it began cutting out and surging back repeatedly. you can also give it some throttle in neutral, and it revs up, then cuts out, then revs back up. it also will not idle now for long before going dead.
thinking it was a bad new carb, we swapped it out and put the old one back on. now with the old one on, it is doing the same exact thing. so, it is not the carb.
i checked the spark plug wire, making sure it was still snug on the top of the plug.
at this point i am not sure what i need to check next, does anyone know what these symptoms usual mean on this bike?

thanks for the help.