A wire would work ok if it was the proper wire. I have some extra house wire, the solid wire stuff. I took about 18 inches of some of this wire and stripped out the bare ground wire. I then used a spark plug and wrapped a few inches of the ends around the plug. If done right the plug can screw out and back in. I use this long piece by unscrewing both plugs out of the engine, run the wire across the top of the engine so I can screw a plug in each end. Hook the plug wires to the plugs to check for spark.
Now in your case you can use a shorter piece, about 4 or 5 inches long. Again wind the end around a plug so it can screw in/out. On the other end you can make a loop that fits under a nut or bolt.
Now put this under a bolt close enough to the plug you want to isolate. Pull the plug from the engine, thread into wire, plug the wire on. This will hold the plug grounded and in a solid mount so you can run the engine.
I made a set of short adapters from nails. Find some the same size as the threaded portion of the spark plug. Thread one end on the nail to match the plug threads where the wire plugs on. I had some bits of 1/4 thick aluminum that I cut a couple strips out of. Drilled the right size hole through the aluminum so it can thread on the plug, thread the nail in the aluminum.
You may need to cut the nail shorter. Try plugging the plug wire on the nail, cut as need to put the plug wire on but no so short that you can't see some of this adapter.
Now make up or buy a jumper wire with alligator clips on both ends.
With the adapter in place on the plug start the engine, hook one clip of the jumper to a fin on the engine. Then hook the other clip to the adapter. This shorts the spark to ground through the jumper wire.
This "kills" that plug so you can adjust the other side.
If you have some bad spokes from an XS wheel the threaded end is the same as the threaded end on the plug and you use a short piece of the spoke and the nipple as the adapter I described making. I made mine because I don't have any XS spokes.
Leo
