Posted by LJD on November 11, 2011 at 05:15:14 from (67.142.130.11):
There was recent discussion over at the "stationary engine" forum. I'm looking to expand it here where more might read and give input.
Generally speaking (if I've got it correct) a waste-spark-system uses one controlled input at the primary coil winding, but the secondaries have both ends of the secondary coil hooked to two spark plugs. I've got a Shaw tractor with a 2-banger TE Wisonsin engine that uses this system. Also a French car (Deux Chevaux AKA 2CV). Also an Onan garden-tractor engine and also a 1973 Honda motorcycle.
Waste-spark makes a spark on both spark plugs but since only one cylinder is on the "compression stroke" I assume the other spark is called "wasted?" I have a 1979 Datsun sports car that has a Jetronic fuel injection that seems to do something analogous. It injects a half-charge of fuel on every stroke at every cylinder even though it's a four-stroke. Runs fine, though.
Back to "waste-spark." I've had a problem with several of these systems for years and thus my question. And it's not just me. I was just reading a long similar discussion at an electrical engineering forum and a engineer has the same questions as me.
So, for example - take my 1973 Honda, CL-175 motorcycle. Two cylinder four-stroke-cycle engine, one set of breaker points, and two outputs from the coil to the two spark plugs. When the bike was new it started and ran fine. But once it got around 20 years old, it go so that when first started and cold, it would only run on one cylinder. And it was random. The dead cylinder was not always the same one. So, when first started, all I have to do is pull plug wires, one at a time. If I pull the wire off the cylinder that was firing and pull the plug boot maybe a 1/2" from the plug - a long "lightning bolt" type arc is created and THEN both cylinders fire. It's been this way for years now. Once warm it runs fine and I stick the plug boot back on. I've put in new plugs and a different coil and it made no difference.
Now, to expand. At this electrical engineering forum, one engineer is having the same issue with his two-cylinder car. When it misfires on one cylinder, he says he can totally unhook the other (running cylinder) and then the dead cylinder comes to life. He wants to know how this is possible since . . . he claims one end of the coil secondary has no path to ground. My guess is - that the coil has made a "short" to ground that he's not seeing.
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