My friend has a 08 plate runner 50cc, When he starts it it makes a rattling/knocking sound and when you pull back the throttle it has almost no power, max speed is about 12.
check the transmission, my runner 125 had a rattle, turned out my outter pulley had come loose and was grinding on the kick start!!! pop the transmission cover off and check, this would explain the low speed too as the outer pulley may have moved away and the belt can no longer go up and down the pulleys
