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There's another old-school test sometimes called a power-balance test.

You one-at-a-time disconnect each plug wire and see if the RPM drop is the same. That'll give you an idea if all cylinders are contributing equally. If the RPM drop is the same, then I'd say the temperature difference is a red ...

I'd do a compression check now

In the Pinz I run a pair of standard Pb batteries (matched). But I do run a separate LiFePO4 battery for my 12V system that powers the ham gear, and that is kept up with a dedicated solar charger which works nicely for the purpose. That way I don't introduce switcher & alternator noise into my radio ...

I have a similar experience of the gentle idle-down to something around 900RPM, and then lower if I blip the pedal. Is this normal for the 710/712?

Can you show us a pic of where you're doing the measurement? Presuming you're using a non-contact IR-type meter, it's important to make your measurements all at a similar distance so size of the sensed area will be similar.

I would swap spark plugs between two cylinders and see if the symptoms move ...

All (with a few rare exceptions) Li "12V" batteries have integral BMS (Battery Management System) electronics in them to protect against:
- OV (over voltage)
- OC (over current) discharge and charging
- UVLO (under voltage lock-out)
- under and over temperature -- mostly for charging

Most of these ...

If you run standard lead-acid batteries, it's perfectly fine to charge them like the Pinz does in a series configuration. But there's a key rule to that: The batteries MUST be matched. That means replacing them as a set with the same batteries at the same time. That's going to give you the best ...

$100 donation sent. Thanks for keeping this going.

...also, check fuses/breakers.
Check that the shutoff solenoids at the carb bases are powered. A wire could have failed

The Pinz is a simple caveman engine. You need spark and something for the spark to ignite (gasoline vapors).

I'd first pop off the ball-joints that go from the linkage to the carbs. See if the hang-up still occurs. That way you can isolate it to the carbs versus the horizontal linkage to the pedal.

I run Phillips 66 X/C aviation oil 20w50 in the plane and am tempted to do so in the Pinz, but simply haven't. I've looked for information about zinc content, and don't see any. The regular oil analysis widely varying numbers, so I don't know what to make of that. They range from 1 to 18ppm zinc ...

I'll add in that piston aviation is still mineral-oil based -- no synthetic. And most all are pushrod engines.

Also, zinc levels were apparently reduced for increase compatibility with catalytic converters -- not an issue for our vehicles, so we could freely bump up the zinc.

Interesting (to me ...

Yes! I'm waiting for the final assessment from this guy: https://youtu.be/a3uXSI9wQv8?si=1BEAXtu6UdUgOST9

He's reset my thinking on using Rotella in my older trucks: https://www.youtube.com/watch?v=HcJHJL5Ys-Y&t=757s
And here: https://www.hotrod.com/how-to/diesel-oil-in-gasoline-engine

Science ...

You turn them 60-degrees past contact, not 90-degrees.

IMHO, the bolts are fine to reuse so long as you know they've never been over-torqued. If the bolts haven't haven't been taken past their elastic zone, I don't see why they'd be unfit for reuse.

Of course, that's always an option. My intent is to try to avoid opening any connections.