Try making a new temporary earth from the tank to negative battery terminal. If it still doesnt start you know you can rule that out. But if fuels getting there, air isnt usually an issue then its gotta be a spark. Take a plug out, hold it with insulated pliers against the head and get someone to turn the engine over, make sure you're away from the plug hole in the head cause if it does spark you could end up with an engine fire.