A mechanic told me that the car battery will send too much power to the motorcycle and can break parts or even cause a fire. He says that I've just gotten lucky the times that I have done it. I'm an electrician, but not with DC and car battery type stuff, of course.
He says it is also bad to jump start a small car with a big pickup truck for the same reason, just not as bad.
He says it is also bad to jump start a small car with a big pickup truck for the same reason, just not as bad.