How can I fix my Sears Craftsman 6.0 quiet lawn mower?

I have had this lawn mower now for 5 years and never had a problem with it, until now. Before I mowed my lawn yesterday the only thing I did was put gas in it because it needed it and then proceeded to mow the lawn. I noticed that it was making a loud ticking sound coming from the motor. It seems like it's putin and has little compression. It was also leaving alot of grass behind after I cut it instead of going into the bag. (the bag was not full) I press the red button on the side where it revs it up and it sounds fine when that button is being pressed. Also I noticed it was blowing a little bit of black smoke from the side. As far as cutting the grass it does that. Just wondering if it's a part that I should replace or something else that I can do to fix it. The mower is a Sears Craftsman 6.0 quiet lawnmower. Thanks for any help.