Also there is this forum Norm's 2 stroke repair center - Motorized Bicycle: Engine Kit Forum
That Norm has done a LOT to educate people about these little motors, he used lots of pics to help the learning process. Poke around in that forum and I think you will find the answers you are looking for. I also think you will find answers to questions that haven't occurred to you yet.
Good luck and welcome to the greatest addiction yet.