This isn't about watching how much the voltage drops at the battery when you use the starter, it's about measuring the voltage drop through a circuit looking for bad connections. The starter circuit seems to be the one with the most problems so I'll show you how to check it. The procedure will work with any circuit though.
You'll need your trusty digital voltmeter, analog meter might work but a little more difficult to read for this test.
First you'll want to make sure the battery is in good shape and fully charged. Disable the coils so the engine won't start.
Yamaha states that a starter circuit with a voltage drop of more than 0.6 volt has a connection or wiring problem. What you really want to see is voltage drops of 0.1-0.2 or less depending on the circuit.
Voltage through a circuit will take the path of least resistance and when your voltmeter is hooked across a circuit with a bad connection it becomes the path of least resistance and registers the voltage.
To check the complete positive side circuit to the starter hook the positive lead of the voltmeter to the battery post (not the connector) and the negative lead to the starter post. Crank the engine and check the voltage. If it's 0.6 volts or less then that circuit is in good shape. The lower the voltage the better. Now do the same with the ground circuit.
If both the circuits check okay and the battery is good then a slow crank problem would be with the starter providing of course there's not a mechanical problem.
Let's say though that the voltage read, for instance, 2.0 when checking the positive circuit. What that means is that 2 volts of whatever is available at the battery is not reaching the starter. You can find the problem by checking the drop across each component in the circuit. The easiest of course would be checking it between the battery post and the cable connector. Then from the battery to the relay post and then the other relay post, you get the idea. Somewhere along the line that 2 volts is going to show up even if some of it is at one connection and some more at another.
If you install a heavier starter and the voltage drop in your circuit is still less than 0.6 volt then the stock wiring is okay. The stock relay is rated for 100 amps and the stock starter is rated at 800 watts or 67 amps Although I don't know if that's the locked rotor rating.
Hope this helps.
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