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I'm not up on the Onan model numbers, but if anyone is talking about
the old opposed-twin flat heads for a backup generator, I wouldn't
think those would be a great choice except for low-use applications.

OLD? They made them for a long time. Don't know when they stopped. If age
bothers you, find a newer one, but some of the older wones were the best
IMO.

My neighbor was fond of them because he could get them for peanuts,
which was a good thing because he seemed to have gone through quite a
few. They didn't seem especially reliable, quiet, or fuel efficient.

Sorry, I don't know anything about your neighbor so I can't argue about
that, but I can tell you about my Onan because I have known it since it was
new. It was perhaps 30 years old when I bought it from my employer. Except
for sparkplugs and batteries, it was all-orignal. Nothing had ever broken!
To be honest, after I bought it, I did have some problems with points.
Perhaps the problem was the mechanic (me). Anyhow, I solved that issue with
an electronic ignition conversion.

I'd think that the fuel inefficiency alone of flat-heads makes them
uneconomical for extended use.

There is a germ of truth here. We are talking about something that was
designed when gas cost 50 cents a gallon! There are certainly more
efficient generators around, but they are not cheap and you are unlikely to
find them at Home Depot. Fuel cost is important (OK, damn important), but
it is not the only cost of running a generator..

I'm not even sure that the 1800 RPM is
any great advantage

Running at 1800 RPM (vs 3600 RPM) drastically lowers the pumping loss of the
engione (increasing efficiency), greatly decreases noise, and reduces wear.

Better 3600 RPM engines last a long time if properly cared for,

"Better" (commercial quality) engines are expensive and are not found on
consumer-grade generators. I expect my Onan to outlast me. I can't say that
about any other small engine I own.

and one can throttle most engines down to a lower
speed if planned for.

No, you can't throttle a conventional generator down to lower speed if you
want 60 HZ power. Running at 1800 RPM (vs 3600 RPM) drastically lowers the
pumping loss of the engione (increasing efficiency), greatly decreases
noise, and reduces wear. Yes, you could design your own generator with a
throttled-down 3600 RPM engine and a belt ratio to get the proper frequency,
but you would no longer get rated power from the engine.

Finally, these (Onan) generators are made for motorhome use. They must fit
into a minimum space, must have reliablility comparable with the vehicle's
engine, and must run with minimum noise and vibration. My neighbors don't
even know that I have a generator because they can't hear it!

Vaughn