Starter issue

Hello,

I have a Briggs and Stratton engine, 15.5 HP, where the starter refuses to turn the engine. I have the spark plug out, drained all the oil, just to make it easy to turn . Also, rigged the wires directly to the starter (positive to the starter, negative to the body of the engine). The simptom is: it starts, it rotates, but as it touches the gear ring, it does not have the power to turn it. A curios thing is that if I use the continuity on the multimeter, the positive side of the starter is making a path to the ground of the engine, I guess that the started is shorted?