I had the same problem once, turned out I had the wrong "start-up" procedure. In other words, I was unknowingly flooding it. I left the choke open and opened the throttle about 1/4, cranked it over and it started. Try some different things with the carb. as far as choking, throttle position. But first I would try starting it with some starting fluid. Easy test to see if you have any fuel delivery problems. Not saying that it isn't an ignition problem, just another avenue to explore.
