It "bucks" so it is getting spark and some fuel. As it then fails to start, begin by checking its timing with a timing light.

You have now performed an easy test which results in a quick clear answer which will usually either find the problem or tell you the ignition is ok.
[Note that an intermittent connection to the coil, points or ballast resistor can keep it from starting even if timed properly.]

If its ignition timing is ok, and the wiring is ok, then move on to other things which could be wrong.

You could put some fuel on a cloth around the aircleaner, and if it starts and runs a few seconds you have found a fuel problem. This procedure is dangerous, so do it outside and have a fire extinguisher there(preferably C02 as that does not dirty up the engine).