idles but will not rev

[clueless]

49cc taotao will start right up but will not rev past idle. when you twist the idle you can hear a change in the engine but stays at the same rpm. I have cleaned the carb, totally dismantled and rebuilt. put it back on and no change. put new carb on, same problem. completely tore down the new carb to check for issues, put back on and same thing. moved to valves, set and same thing. replaced the CDI with three others and nothing changed. I am beside myself and he doesn't know what to think of it either!!! can anyone help? thanks!