Distributor, or coil? If the coil is sparking from anywhere but the spark output terminal, it's faulty. Insulation has broken down somehow.
Yes. Faulty insulation. But if the coil sparked out the side ALL the time...the engine wouldn't run at all. Seems like only one or maybe two cylinders have enough spark voltage to escape through the side of the coil.
I just love using an automotive 'scope on ignition systems. They'll show differences in cylinder-to-cylinder ignition problems quickly and effectively.
Automotive 'scopes are becoming a lost art, largely because distributors are an endangered species; and other ignition types--waste spark, coil-on-plug, etc. are not as easily connected to a 'scope. (They CAN be connected, but often each cylinder separately; and it's a pain in the tuckus and takes a long time.)
High voltage will eventually destroy the insulation, but it typically takes awhile.
Yep, I can imagine that. Never tried it...but I will. I learned something from this. I've just attached the spark tester to the coil, and then grounded the spark tester with a jumper wire.
Old-style coil-in-cap HEI distributor--but connecting to a separate coil or the coil wire works as well.
You must be registered for see images attach
You must be registered for see images attach
Really, enormously high resistance in the plug wires will sometimes do it. Not often, though.
Typically it's an excessively-large gap somewhere in the system. Spark plug gap being too wide is common. Rotor tip-to-distributor cap terminal is less common.
Depends on the plug wire construction. Anywhere from ~35 ohms per foot of wire, to 4,000 ohms per foot. I'd be suspicious of anything beyond 4,000 ohms per foot, but I've seen plug wires on running vehicles that have 20,000 ohms per foot; and plug wires with ~100,000 ohms--but the cylinders were misfiring badly.