Not your normal bosch pump thread

[Dave]

The small pipe is connected to the govnor, which basically allows the pump to advance based on the throttle position. The one without it (with just the silver cap) has a linear advance based on pump rpm so does not take into account throttle position.

I think the solenoid is just there to stop the govnor affecting the advance during cold starting as it can back off the advance when no throttle there so can be hard to start without it (though in reality makes no difference) - I *think* this is what the electronic valve is for but 100% on this.