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

Years ago when i purchased my M90, i experienced the same sort of problem once...
The engine ran flawlessly, and then suddenly died, and was impossible to start again after.
It turned out to be some residue (maybe from manufacturing), that had placed itself on the exhaust valve, keeping it ever so slightly open at all times.
The fuel system is as simple as it gets on these, and the minute loss of vacuum from the non-sealing exhaust valve, was enough to prevent a proper intake of fuel.
As soon as the exhaust valve was re-seated (by merely flicking it a few times, and turning it around in its seat), the engine ran again...and has done so ever since.