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

Does the piston feel like it has stiff resistance when turning the flywheel?

If not, I'd suspect that it is the ports that are clogged, possibly with flakes of hard water deposits, or even gunked up with old steam oil.

Try some solvent followed down the steam chest inlet and work the flywheel around several turns both directions.

Have you checked the whole in your banjo bolt?