Confining the magnetic field of the solenoid motor stator is a significant improvement. The force increases and hence the rotational speed increases.
This is the first of a number of improvements to the solenoid motor. However, as you can see from this video it is a significant improvement:
Here I have just used 0.9mm thick mild steel strips that are ~5mm wide. Effectively these strips guide the magnetic field and increase the strength of the magnetic field by increasing the density.
Next I need to look at ways of making this an interesting feature.
This is a lovely item to make and run, a mix of wood and metal. The engine is a great executive toy and looks in keeping on any desk.
PDF download of the plans for this simple engine.