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I get a reading of 3.9 to 4 volts measured at the starter solenoid when the starter is engaged. That seems a little low to me. I tested the voltage with the engine cold, about 55F, clutch engaged, with 20/50 oil in the crankcase. Age of the battery is unknown, but it look OK and is clean with clean connections and good cables. The charging system appears to work very well, with good regulation. Is 4 volts on the low side when cranking in mild weather?

What is the voltage across the actual battery terminals when cranking?

And more importantly what is the voltage at the coil terminal being fed from ign switch while cranking. 4 volts is low and the spark is usually very weak and often can't jump the plug gap when under compression if the voltage at the coil is much less than 5v.

Quote:

Yes, I would expect at least 4.2V.

I would check the voltage across the battery. Then I would check the voltage between the battery post and starter terminal (not the battery cable lug on the solenoid) when cranking. Should be very low, maybe 0.1V. An excess drop indicates either an undersize cable and/or a worn out starter solenoid.

Thanks. I did test at the battery side of the solenoid and got 3.9 to 4.0 on a Fluke digital meter. Didn't check the starter side, which I will do, and I haven't cleaned or tightened the ground cable to frame connection.

Are you using at least 0 AWG cables?

Have you run a load test on the battery?
You can get a load tester relatively inexpensively. If you don't want to add a battery tester to your tool collection, some auto parts stores will test your battery for you. They, of course, hope to be able to sell you one.

I’ve had batteries tested at Napa and a proper test requires leaving it over night.

The only load test I have is the starter. Testing the voltage at the battery posts while cranking may reveal a weak battery. If it's about the same at the battery posts as at the starter side of the solenoid, it would point to a weak battery or a worn starter. If the voltage is much higher across the battery posts than at the starter when cranking, it would point to a high resistance in the cables, solenoid etc. It always starts, but it does not crank as briskly as it did a year ago.

I would say that battery is starting to go. I would get it tested.

Generally if there is another problem in the system resulting in poor cranking, the battery voltage when cranking will actually be a little high since there is more resistance in the circuit, and hence less current draw.