I seem to have missed your post saying it did start with the starter fluid. You're sure about spark and fuel so next is to check air.

Check the AFM plate if it visibly goes down when you turn your key to Key-on-engine-off position. Also have some body start the engine and while cranking slowly push the plate down a millimeter or two. It happened to me once where I over adjusted my fuel mix and the engine was not getting enough air. It can also happen that the engine is getting too much air.