HT electrical puzzle.

[Greg S]

A Question for wizards. Automotive wizards that is. Automotive HT electrical wizards at that.

Here’s the situation. 4 cylinder carbureted, points and coil engine. (Not Land Rover but probably doesn’t matter.) Has wire core HT leads that are old. It was driving down the road and quit. Filled it with gas thinking it had run out. It still wouldn’t start. Pulled plug wires off and stuck them back on, both coil and plug ends; DID NOT get wires mixed up; these were done one at a time. Still wouldn’t start (imagine that?). I said it was flooded and as there is one cylinder that is prone to poor running and plug fouling, I directed the work crew, my son David and his friend Derek, to pull that plug first and dry it. Plug wasn’t too bad but when dried, we stuck a lighter in front of the open hole to burn off excess gas (Children! Listen! Don’t do this at home! Only an idiot would do this.) The gas burned off uneventfully for those of you who are worried. It’s real neat when you turn it over when doing this as a flame comes shooting out a couple feet; any way, dried and de-flooded. On to the next cylinder and did the same. Left the plugs out to turn it over and get clean dry fresh air inside. Darned if it didn’t fire on the other two cylinders and start running. (Yes. It was a bit rough.)Shut it off, put in the two plugs back in, stuck the wires on, and it wouldn’t fire. Pulled the wire on the bad cylinder and for some reason cranked it while the wire was OFF, and it started. Put the wire on while it was running and it stopped. Wire OFF and started it. It ran fine considering. As the wire approached the plug cap, the spark was half an inch long or more and it started running on all four cylinders. When the wire was stuck back on the plug, it quit running. Wire off it ran, big spark jumping across, it ran great. Took the wire off the dizzy, put it back on the plug and got a great spark between the dizzy and wire end, motor ran great----till stuck back in the hole----- when it would shut off. Here’s an oddity. HT wire OFF the plug and hanging in mid air, close to nothing. Wire off the Dizzy, approach the dizzy hole with HT wire and get good, long, regular shots of spark going to the wire---But it isn’t connected to anything at the other end. Replaced the wire with a different one and the engine still shut down when that plug was hooked up.

Well? What? Why? How?

[John]

You're only going to get spark if you have a path to earth (ground). If the other end of the HT lead is not connected to the plug then somewhere along the lead you are going to earth. How are you holding the lead? Sounds like it is time for new leads and maybe plugs.

[Greg S]

Plugs are near new. Wire is solid core but caseing must be shorting somewhere. We were just holding bare handed with no tingle at all.

The problem was a crack and tracking in the cap. When the wire was off the one plug, there was too much resistance for the spark to ark across the cap and go to ground through the por wire. When the wire was attached, every spark seemed to go to the one plug ar the resistance was less than jumping the gap at the end of the rotor.