The engine are poorly assembled, most often the engines are older types with bushing's instead of bearings, low quality material, ever so often someone gets a good one but not often, get one if you truly believe it's quality but do some R & D on this forum first and 89.9 precent of people will tell you not to get it, if you are getting it for a dirt ball price don't be deceived THEY'RE CRAP !!!
2 stroke Engines> PK69.5/80cc< you can't go wrong! you will almost never hear of someone getting a bad PK 80 and most performance parts are for Grubee and PK 80's ( Power King)
If you look to the right of this thread you'll find a vender named DAX they re-make a fantastic! 2 Stroke F-80 at a decent price with Balanced crank and other HIGH QUALITY parts again no one complains about their product either.
Their 4 strokes are a Dammnn! Good ! price and they stand behind the merchandize and $$ you spend, A real honest vender