What does your volt meter read across the 2 hots? I'm not talking the hot to ground potential. On the same 2 hot leads what is the AC frequency? Are you actually getting 60hz? I am assuming a 3 wire receptacle on the Genny. Is it 6-50r to match the welders 6-50p? Or do you have an adapter of some kind?
My meter won't read across two hots, but both hots register to be the correct voltage. I have a 10 gauge cord coming from the L14-30 receptacle, and an adapter that changes it to 6-50. Got it from Evseadapter.