Spark Plug 101
Easy check is to get one of those little neon light doo-dads that electricians use to check for "live" circuits. Hold it right against the spark plug wire. All of the plug wires should show about the same intensity. This will only tell you if the "system" is delivering voltage to the plug.
Alternatively, you can pull the plug and "ground" it to the block and crank the engine and see what kinda' spark you're getting.
If you are not getting a healthy spark, you can swap in a "known good" plug and see what happens.
Plugs can be bad internally also. The spark will jump inside th plug intsead of at the gap. I once cleaned up an old straight-six F100 Ford pickup engine with a heavy cleaner/degreaser (Oakite, IIRC). Not thinking, we left the plugs in the engine. When we tried to crank it, we couldn't figure why it wouldn't fire for a while. All the plugs had shorted internally due to the cleaner penetrating and providing a low resistance path. (It was easier for the spark to jump through the Oakite residue inside the plug than across the normal gap at the end of the plug. Once the spark started jumping internally, it built up a carbon path that was almost a complete short) A craked porcelain insulator will usually wind up exhibiting the same problems.
Also carefully examine the distributor cap for carbon traces (minute, spidery tracks) in the vicinity of that #5 lead.
If the plug is good and it's still not getting an arc across the gap, swap out one of the spark plug wires and see if it corrects the #5 problem.
All of this can be done in a few minutes. From the get-go, you have five good wires, five good plugs, and five good distributor outputs to experiment with.
john
