"The old" ones are inertia starters (rotation of motor shaft causes the starter dog to throw out)Well thats a mine field trying to find another starter motor. Finally found one on EBay thats done less miles than we have. Think its from a breakers. So now can swap over and see if it eliminates the problem. As others said need to add another earth connection. Will also find out how solonoid works as it looks like it throws the cog out and connects. Not like the old ones with a spiral shaft. Anyway something to play with.
And usually used a separate solenoid
The ones with the solenoid piggy backed on the starter are referred to as 'pre engaged'
The solenoid moves the starter dog into mesh with the flywheel ring gear before completing the electrical circuit to turn the starter motor.