These machines are rubbish (I have learnt the hard way!). Just google search it and you will see what I mean - I am not the only one who has had the mis-future of buying one of these and living to regret it. The likely cause here is that the ignition timing ring has slipped and so the spark is not happening at the right time and hece the motor will just about start but will not rev. This is repairable if you want to strip the engine apart but the repair will last for only a short while and then it will happen again. My advise is throw the useless piece of rubbish in the bin and save yourself the hassle.
Blewitt, September 2013