Battery Voltage Low - most likely culprit?, Does not start well when warm Options

[Chaznaster]

Hi All.

I am having a trouble starting my carbureted 1.8 when the engine is up to temperature (heads at 310 degrees for over 30min). The engine is definitely turns over and the starter engages.

The car starts immediately when cool/ cold. It is also fine when I let it cool after being hot.

My battery seems to have settled out at about 12.2V. Per the Google, that equates to 60% useful charge. That is definitely a problem.

My theory on the non-hot start is that, as copper wire resistance increases with increasing temperature (also something learned on Google), it is just not getting enough current to the coil when hot. Cold wires = lower resistance = better current flow, and so, even with the marginal battery, it gets enough juice to start.

Now on to why the battery is running low. At warmed-up idle speed (800-900 RPM), the multimeter only reads 12.3. That is most certainly the cause of the low voltage.

I'd very much like your input as to whether this is likely caused by a voltage regulator problem or an alternator problem (or both).

EDIT: It will always start with a push start ...

Many thanks!

[Superhawk996]

EDIT: It will always start with a push start ...

You’ve got multiple problems going on. 1st not charging properly.

Check the voltage at the coil - when it is hot, and while you are cranking . I suspect you are going to find the voltage is very low while cranking. I’d bet that it’s probably 9v or less and you are getting a terribly weak spark; not enough to light off; especially if mixture is already a bit rich when hot.

When you push start, the starter motor load doesn’t drag down the voltage at the coil and you get a decent spark, enough to start it.

Measure coil voltage while cranking as was suggested by Brant. Fix charging problem. See where you’re at. There are plenty of other diagnostics that can be done. No need for guessing.