Generator question?

If it's a 12 lead reconnectible type there are a number of possible configurations. Check on the Onan or Kohler sites and you should be able to find and explanation of the connection options.
Basically for each of the three poles you have two sets of windings brought out separately. Connect parallel for low voltage, high current or series for high voltage, low current, can connect on delta or Y configurations and an odd "zig-zag" configuration to get single phase.
Pete C.

If that's the typical Hercules flat-4 gas (spark) engine, as commonly seen in the mil-surplus gensets; it is a GAS HOG. And noisy too.
If you're only running it an hour here, an hour there; then fuel consumption obviously isn't an issue. But for heavy or daily usage, you'll find the $$$ mounting up fast.
And by the way, Hobart was one of the mfg's who supplied those commonly-seen mil gensets...
Hercules made a series of inline diesels too; which were fine little engines. If it's one of those, it's a gem.
You already got a reply regarding the wiring. If it's a 12-lead alternator; it's 99.9% certain to be a 110/220 capable unit.

Can I take a guess, is it a MEP-018A?
If so, I had one similar (MEP-017A) briefly before I resold it. It is a gas hog, like someone else said. Also very heavy. The plus is that it is utterly reliable and will last forever and starts in bad weather. For 3 phase, it is much better to make a phase converter than it is to run a genset.
i