i had this same issue straight out of the box with my Z80 from Boygofast motors on ebay, i would bump start the engine via peddling ride for a split second or two then VERM VERM! sudden extreme rev's in RPM followed by the engine killing off,
i first tightened my Cluctch cable ''witch is clearly the first thing to try''
and attempted another run with no change, it went into high rpm range then shut off,
i pulled the clutch case and took a look... all is good in clutch case.
it turns out that under the Smaller sized gear if you remove it ''drive chain gear?'' anyway if you remove the gear there is a groove cut into the shaft witch it screws too,
there is also a groove in the inner hole of the gear its self there *SHOULD* be whats called a Woodruff key in there witch looks like this
http://www.chinesemotorcyclepartsonline.co.uk/images/largeimages/120905004.jpg
i only know this cause when i opened the crank case it fell to my feet lol it was some what of a pain to wedge back in there, in fact i tried a few times, the first three attempts i did not get it all the way snug and fit,
So when i went to and attempted a test run it cranked over then seconds later reved up and shut off again too my displeasure lol, so i opened the crank case and the little bugger fell to my feet again and the small gear had once again came loose!
if you do not get the woodruff key snug and proper, because you tightened the smaller gear witch locks the key in place it had enough grip too crank over
yet when it began too hit any type of high or mid range rpm
the clutch would slip causes the poor little ole 2 stroke engine to rev up wildly then sputter out like some one hit the kill switch,
just get the woodruff key back in place tighten the smaller gear back down and you won't have to stress the issue again!