Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The engine has a definite knock. While stationary determine which journal is sloppy.  I like to take very thin clear plastic to make up the slop. The kind from blister packaging.  Better would be very thin brass sheet or foil.

I suspect your piston rods are worn. A way to check for that is thick steam oil. Dripping some on it, will improve the running momentarily.