ORIGINAL: AA5BY
One point not made yet is that ABC engines don't tolerate being run rich as the liner must heat up properly to provide the necessary running clearances but engines with bushings kinda like a little richer mix as noted by several comments above. For this reason I'd not choose an ABC motor without bearings.
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There's only a few clicks of needle adjustment between rich and lean so I really don't think the bushing will know the difference. I think that mixture is used to fine tune the ignition timing of glow engines. This is based on my experience with changing the compression ratio of glow engines and noting that an engine with a lower compression ratio makes it's maximum power with a leaner mixture than the same engine with a higher compression ratio. The overheating of a lean mixture does not come so much from lack of lube but from the mixture igniting too soon as the piston moves up the bore.
Also, remember that in ball bearing engines, the con-rod still uses a bushing.