Hastely rushed production at the factory is partly to blame for a lot of thing's too,
like unbalanced cranks, bent connecting rods and bearing races, poorly cast cylinders, old milling machines contribute to a malfunctioning engine, rolled bolt threads instead of cut threading, low grade steel and aluminum, basically in China all you need to assemble an engine is a pair of pliers, hammer, screw driver, the torque on some of the heads of the engines I've re-build have been 20 ft lbs some you could untighten with yer fingers.
Some are better than others I have found that the PK 69.5/80 cc and Dax engines are the BEST! and require the least amount of work, re-engineering and or mods in my opinion Grubee is in the mid range with quality declining in the past 2-3 Yrs. you either get a good one or a crappy one, better know vendors in the United States have required that the producers of engines build and ship a better product and slowly the Chinese MFG's are getting the point.