Can I get 240v out of a 120v generator?
What you describe won't work, and it's not safe for you (or anyone else) to do what you describe.

If you have a single phase 120 volt generator and you need to supply power to a 240 volt split phase load, the safest way is to use a transformer with a 120 volt primary connected to the generator and a 120-0-120 volt secondary connected to the appliance.