I have to date fitted six motors for myself and other people so I have some knowledge on the motors with some common problems showing up.
All of the chinese motors have poor electrical connections from the motor and coil. The round push in contacts are not good enough for the purpose so after a bit of use they fail to make a contact and even come appart. To be sure of a constant connection I cut off the terminal, bared about a cetremeter of the copper wire and soldered the wires together. Tightly wrap one wire over the soldered joint with electrical tape then the other. Shrink tube would do the job just as well and look neater as well.
Chain stretch, especially on the 80(66cc) motor is a problem but can be overcome with the fitting of a heavy duty and better quality chain.
The fitting of a small in-line fuel filter is essential to get clean petrol to the carby, the internal filter above the petrol tap in the tank simply only filters out rocks, the metal mesh type filter is better than the paper type and worth the extra cost and effort to find.
If you have a problem drop me a line with ALL the information you can think of and I might be able to help sort it out. I have also done a fairly expansive engine tuning exercise on my own motor with good results in low and mid range power, some is dead easy to do and each part can be done on its own to get a better result from the motor.