According to the site Tritium pointed to, Neutral is the jumper between U3 and U4.
Are you using the genny for a 240v load, too, or just 220? If just 220 you can boost the current by reconnecting the genny so the two output windings are in parallel rather than series. From the diagrams that's U1&U3 jumpered as one side, U2&U4 jumpered as the other. But try it by jumpering U1&U3 then checking that you have 120 between that and either of U2 and U4, and only a couple volts between U2&U4, before closing the jumper between U2 and U4.