If it cranks slow, that's the first place to start. If the voltage drops below 10 volts while cranking you wont get enough voltage to the coil for a strong spark. If you put jumpers on it and it fires right up then look at connections to the starter and ground. If all those are good do a voltage drop test to the starter. Connect volt meter from battery+ and to starter terminal. Crank engine and record voltage, Should be near 0. Anything above 0 indicates you have High resistance in the starter circuit.