All plugs were out. Key was off. The plug was being installed on a 350 at the end of a rubber vacuum tube to get it started.

Any ideas on what the cause would be? I'm stumped!

 

OK, I've been under the car to see if the starter wires are shorting out or oily and causing me headaches. I moved the wires around, but did not find any chafing on the wire insulation. Now that I have had a chance to think it through though, I am thinking that the cause may not be the spark plugs. I am thinking that the spark plug wire shields are to blame. They are a bit loose, and I think that I, in the process of searching for the cylinder hole, had wiggled the shield's sharp back edge into an unseen loop of wire going to the starter solenoid. It's pouring now, but I'll go check this theory later.

 

While you are under your hood, get rid of the PLASTIC fuel filter and replace it with a metal enclosed unit. A plastic fuel filter is not designed for pressure applications. They are fine on the suction side of the fuel pump.