weird spark problem

[chris myers]

I've been runniing on three cylinders. Started pulling plug caps to find the dead cylinder, got the cap to the top of the plug with spark jumping to the plug from the cap. The cylinder will then start firing. It does it every time and stops firing when I put the cap back on the plug correctly. Any ideas? It's a 78 GS750E with new dyna coils and matching ignition, new wires, plugs and caps. Thanks in advance. This turned out to be what I had thought was a carb problem.
Chris

[nabrams]

There's a simple test you can do here: Swap that "suspect" cap to a different wire and see if the problem follows the cap. If yes, then get a new cap. If not, then swap wires. If the problem follows the wire then get a new wire.
You get the picture.

[fgh]

I first heard about this on the Wing board. When the current travels down the plug it does not immediately jump across the gap. It builds up then arcs across. If there is a crack in the insulator or deposits on it the charge will leak off to ground as it arrives and never create a spark. When you pull the plug cap off with the engine running you cause the charge to accumulate at the cap terminal, it then jumps the gap to the top of the plug and will jump immediately on reaching the end of the plug as the charge has already been accumulated.

My GS has resistor caps on it and the is also some resistance in the wires. This resistance would also cause some accumulation of the current. If you've got a stock set of coils it would be a good idea to run a resistance test on them to make sure they're in spec. Also check the plug caps to make sure they're the right ones. Some PO may have put on non-resistor caps when you need resistors for it.

[nert]

I first heard about this on the Wing board. When the current travels down the plug it does not immediately jump across the gap. It builds up then arcs across. If there is a crack in the insulator or deposits on it the charge will leak off to ground as it arrives and never create a spark. When you pull the plug cap off with the engine running you cause the charge to accumulate at the cap terminal, it then jumps the gap to the top of the plug and will jump immediately on reaching the end of the plug as the charge has already been accumulated.

The explanation above is correct. By enlarging the gap, pulling the cap a way from the spark plug, one intensifies the spark voltage. I have seen "Auxiliary Gap" spark plugs manufactured with a built in gap for the center electrode down inside the prorceline. They also use to sell a device (that had a fixed gap) that would fit between the automotive coil wire and distributor cap that claimed to increase power, fuel mileage and fire oil fouled spark plugs. I wanted to avoid that last item, BUT your little spark enhancement may be telling you, that you have a fouled spark plug, coud be oil, or could be fuel, or could be low compression.

[rustybronco]

The explanation above is correct. By enlarging the gap, pulling the cap a way from the spark plug, one intensifies the spark voltage. I have seen "Auxiliary Gap" spark plugs manufactured with a built in gap for the center electrode down inside the prorceline. They also use to sell a device (that had a fixed gap) that would fit between the automotive coil wire and distributor cap that claimed to increase power, fuel mileage and fire oil fouled spark plugs. I wanted to avoid that last item, BUT your little spark enhancement may be telling you, that you have a fouled spark plug, coud be oil, or could be fuel, or could be low compression.

DITTO! bad or fouled plug.