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

Hi Raphael,
I guess you checked there is some clearence between the rocker arm and valve when the valve is closed?  May i suggest putting your finger over the inlet and then exhaust while turning the engine, if you feel pressure on your finger and a little compression then you have identified a faulty valve.  If you have a valve issue and clearence is present then it is usually a particle stuck between the valve and its seat.  Sometimes flicking the valve open with your finger so it snaps shut will clear this or maybe flood the engine with fuel and spin it with the drill adaptor.
Hope this helps
Ade