[Brad54]

Actually, you don't need to buy a tool to check for spark: Just pull a plug, and with the plug wire still on it, hold the threaded part of the plug to the exhaust manifold and crank the engine--if you see a spark, you've got spark. If not, you know where your problem probably is.

I always put an inline glass fuel filter between the fuel pump and the carb, as a diagnostic tool for the inevitable "damn, why won't it run now?!" It immediately eliminates (or confirms!) one problem.

This sounds like an over-simplification of things, but it's not:
An engine needs fuel, compression, spark and timing. If it has those four things, it can't help but run... it has no choice. So if it isn't running, you only have to check four things.
An old man told me that.
He also told me "90-percent of all fuel problems are electrical." And I have yet to prove him wrong, on either.

-Brad