Ditch the test light and use an old plug widened to 1/4" gap. Attach to each spark plug wire and ground the old plug to a bare spot on the head or manifold. With the tractor in neutral and the ignition on, crank and observe the spark. It should jump the 1/4" gap with a blue white spark. Report back.
If spark passes the above test, redo the carb. Soak overnight in a can of Berryman's. Before reassembling blow each passage out with compressed air and rubber tipped air gun.