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

Hi Trevor,

My M90 would do this regularly, i would hold the exhaust valve open and blow throught the carb with the airline to shift any dirt, or sometimes pushing the valve open and letting it snap shut a couple of times would do the trick.  Those valve springs are so soft that any resistance will stop the valve seating.
Make sure the valve stem is lubricated with some thin oil too if you have not already.

In the end i went with a slightly stiffer inlet valve spring, the engine ran so much better but sometimes when starting the exhaust valve would suck open instead of the inlet, stiffer exhaust valve spring fixed that and the engine made so much better power under load and generally ran much more constantly.

Hope this helps
Ade