What is the common name for amateurs who maintain and repair automobiles, particularly their own automobile?

The colloquial term for someone who performs maintenance and minor repairs on their own car is "shade tree mechanic".

From the Urban Dictionary:

A person willing to learn and perform scheduled maintenance or simple repairs on their own vehicle rather than being completely reliant on technicians who may be dishonest, careless, or ignorant.

"Many automakers have realized that dealer service is profitable and have designed vehicles to be irreparable by independent or shade tree mechanics."

Yes, the term could apply to motorcycles as in the book by Robert Pirsig.