check voltage at the coils when key is in "run" position -- sometimes they'll get power in "start" but when you let go & it goes back to the "run" position, a bad ignition switch will no longer provide power & they'll die right away. another very similar problem, check fuel pump for power in "run" position to see if the power AT the fuel pump (in tank most likely - you can check it at the wires near the tank) when cranking, and after it starts. if you're not sure about where to check for power on the coil, just take 1 spark plug wire off, put a spare spark plug in it, & start the thing. if it keeps sparking after you return the key to the run position (for the 1 second it keeps running) then you know it's not an ignition related problem, since it's still sparking. at that point, you can pretty much say it's a fuel delivery or ecm problem. put the plug wire back on & pull off the air cleaner. take a squirt bottle with some gas in it, spray a couple of squirts in the inlet & have someone else crank the motor over. when it starts, just before it dies, shoot some more fuel in the inlet. if it keeps running, then you know for sure it's a fuel delivery problem..... most likely the one i said above (losing power at the fuel pump when key is in run position - check for faulty ignition switch).