I have a 2 yr old, 80% gas furnace with a 2-stage blower. I tried to
use a back-up generator to run it during a power outage - but I
couldn't get it to light. The generator is 3000 watts(4000 surge) I
changed the dead circuit from a single pole switch to a short pigtail
cord and connect to the generator with an extension cord.

All the cords are good, 12 ga, and the distance was only about 60'. I
disconnected all the furnace wires from the house wires and moved them
all to the pigtail....The igniter would click, but not light.

I'm told the problem might have been that I didn't connect the grounds
effectively. Is that reasonable?