“….. if not, it would make a serious difference to the machine.” What does ‘it’ refer to?

The bicycle “goes easily enough in the morning and a little stiffy after lunch.”

The remark is

The friend shook the bicycle violently. Find two or three sentences in the text which express the author’s disapproval of it.

Did the front wheel really wobble? What is your opinion? Give a reason for your answer.

In what condition did the author find the bicycle when he returned from the tool shed?