Your kill wire is not connected. We have gone around in circles a bit on that, when you said the green wire was grounded I looked quickly at the picture and thought the same. That seemed to make a sort of sense, since Tecumseh colours their ground wires green. However the wire is actually dark green, and it is actually insulated from the engine - it ends in a standard spade lug. On Tecumseh engines if they have a dark green wire, it is the kill wire, and you stop the engine by grounding it. On Briggs engines the kill wire is black.
If you look at the wiring diagram, you will find there is a black wire that connects to the engine, at a point labelled "magneto". That is the kill wire, and it is labelled wire 8 (Insert edit: it is wire 6, not wire 8) on the wiring diagram. That black wire was connected to your Briggs engine, and it now needs to be connected to the spade lug at the end of the dark green wire on the Tecumseh engine. When you connect it, the engine should stop whenever you switch it off.
Because the sparky apparently told you he'd left out a diode here and there, I don't know what is actually happening with your charging circuit. What does your ammeter (on the mower's dashboard) show when the engine is running? What does it show when the engine is not running?
Last edited by grumpy; 01/07/12 11:13 AM. Reason: Insert edit correcting error
