Cavalier won’t start and makes “plunk” sound

Hello all,
I have been trying to help my dad get his cavalier up and running but we have hit a tough spot. After putting a new fuel filter, spark plugs and new gas in we still can’t get the engine to run. Some when we try to start it the engines sounds liked it is going but it runs really rough and most times we hear a “plunk” sound and it dies.
Any ideas?