Currently pulling out my hair on a 30's magneto for my current project. I have spark, but I'm not sure I have enough. So thus is my question, what is the benchmark of a gap that a spark should jump on a test devise (or old plug with electrode opened up) to be considered GOOD SPARK??? I've heard of 1/8" all the way up to 1/4". Thoughts please.
Here is the pdf of maintaining a Fairbanks magneto. http://www204.pair.com/bbg46/FM Mag Manual/Magneto Service Practices(49-64).pdf I have only worked with aircraft magnetos and old time magnetos from stationary engines, and all I looked for, was a healthy blue spark. I have the information on the amount of voltage vs gap, and I will attempt to find it and post it. Bob
Here is a picture of my spark gap device. I got so I haven't used it much because I got so I could tell how healthy the ignition was just by looking at the spark. I measured the gap on the device, and the range in the green band is from 3/8 to 5/8 inches At the beginning of the green band, the voltage would approximately 17,500, and at the widest, the voltage would be approximately 30,000. It can be a pain in the **** if someone has played with the internal timing of the mag, but even this can be repaired. I hope this helps. Bob
Bear in mind that the coil and etc. was designed and built to resist 'arcing' between the primary and secondary windings......under normal operating conditions. Back when Indy cars had skinny tires it was common [I did it] to pull a spark plug wire and hold it near a ground like the engine's head and then lengthen the arc until it would no longer jump the gap. This was not a 'normal operating condition'. We were [we thought] 'testing' our coil. Nope, we were testing the insulation value between the layers of copper wingdings in the coil. Honest....the spark is going to jump somewhere and a handy place is inside the coil if given no alternative. The first bit of arcing creates a tiny carbon trail. As time goes by the juice may decide to use that little trail from time to time. At some point in the game [usually when the coil is very warm] it totally prefers the now enlarged carbon trail at higher RPM. Ya get a miss and a little reduction in power [and hard starting] for a period of time before that terminal event. Sooner or later the coil will 100% fail. If a coil has been run with a spark plug wire dangling it will surely blow the coil or at least damage it to where it is intermittent in it's operation. Just sayin'.....be careful about making a mag coil or a points coil of a electronic ignition coil do things it was never designed to do.
Modelabc is correct, IF there is a weakness in the internal insulation in the coil a max spark length test may be enough to punch through and permanently damage the coil. If you just test for ENOUGH spark length, say 1/4" (should be plenty on any low-medium rpm & low-medium compression engine, i.e. street engines) you should be fine. That's just mimicing the conditions when a spark plug has to make a spark under compression, at higher pressure the voltage requirement goes up just like when spark length is increased. There is little point of a 1" spark anyway - all that does is arcing somewhere you don't want it to, like to another plug wire connector on the distributor. You want a strong spark, not a welder or lightning machine.
Yeah, the one that makes you remember for 53 years that you don't use a metal pipe to pull the plug wire off the lawn mower to make it quit.
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