Does it never start? Or just sometimes?
When it cranks can you hear it trying to start? Or is it just the starter spinning away?
As for the batteries, I know you mention you swapped them, but did you make sure they were charged first? It might sound stupid but the little things like that will catch you. My ignition box needs a solid 9 volts to start and gives me trouble if I haven't run it in a while or don't run it long enough sometimes.
Not firing is really only ever no/not enough fuel or no/not enough spark or spark in the wrong time. This might be a little ahead of the game but what about timing and such?
