magneto confusion

[tom-c]

working on a two cylinder opposed motor. One coil. Had spark on both cylinders before doing piston work. Motor all back together and now no spark on right side. So i pull both plugs, put a spark tester on the left side and ground the right plug wire. Nothing. No spark on the left side. I unground the right plug wire and get spark on the left side! No matter what I do, no spark on the right side. What am I missing here?

[fleetwin]

Don’t know too much about this style, but am "assuming" that spark occurs on both cylinders at the same time…So, to me, this would rule out points/condenser. I’m thinking the secondary winding on the coil is grounded somewhere so spark is short circuiting to ground before it gets to the left side….
Needless to say, let’s wait for more informed opinions…

[tom-c]

I was thinking the same thing, Don, but it was working fine before the the piston work. I hope someone has a better idea.

[fleetwin]

For now, try checking the secondary resistance of the coil. Then check for shorts to ground on the secondary circuit, I’m thinking there shouldn’t be any…Perhaps one plug lead is chafed/shorted to ground instead of the coil being bad….
Hoping for something simple….

[tom-c]

I’ll try that first. The plug wires are brand new but I suppose they can arc/short anyway. I’m really hoping this coil is still good.

[fleetwin]

Me too buddy, hope you just pinched one of the plug wires….

[Tom]

Tom,

It sounds like one side is shorting to ground. One way to determine whether this is happening is to remove the flywheel and put a piece of paper between the points. Rig up a couple of D cells in series, with wires coming from + and -. (You can use another DC voltage source if you have it, but don’t use a car battery or anything with that much amperage.) Connect one lead to the insulated point side, and flick the other lead on the mag plate. This should be enough to induce a spark at the plug leads. Then try this with your shop lights off. If the bad side is arcing to ground, you should be able to see it. I’ve seen this occur on the primary side, too. If the primary coming out of the coil is routed near the laminations, try moving the wire and slipping some insulation between the wire and the lamination.

Now I understand why you want to get the Merc-O-Tronic working. There is a function on them that would help you isolate just this type of problem.

Tom Manley

[jeff-register]

Tom,
Did you remove the coil from the motor? I have never experienced it but I hear if it was removed it will crack the coil. Try this: Use an ohm meter & ground the mag plate & take the other probe to the both plug leads one at a time. See if the "R" value is the same on both ends. Next connect the meter to both sides of the output of the coil, What do you get? There is an unbalance on the coil somewhere. You can use one D cell for testing too with much less danger to the coil. If you have an extra buzz box from another motor too connect to ground & the low voltage input to the coil with the cap removed from the circuit. The buzz box should already have a cap in the ele system. The buzz box will give you a constant spark. Next ground the output one side at a time. Grounding one side at a time should almost double the voltage. Do you have a meter or hi volt probe to measure high voltage? An a O scope would work the best. I bet you aren’t close to me in Arizona huh? I have all kinds of meters before I did outboards & still do. Please post your results to give me an idea of what is going south on you.
Oh I almost forgot: If you removed the coil go ahead & remove it again being very careful to not touch the bottom. Flip it upside down & with a magnifier glass look for a very small dot of carbon on the side that doesn’t fire. If you see that it may be leaking voltage at that point. If you find a carbon spot clean the area well then coat with Super Corona Dope. Look on E bay for it & don’t pay more than 15 bucks for it. It was designed to cure the leakage from a flyback hi voltage transformer in a B&W T.V. The Flyback upped the voltage to run the scanning beam in the picture tube or the C.R.T. Cathode Ray Tube. Cathode references negative going voltage & Ray= spot of light & tube= a sealed vacuumed tube. The scanning beam can run a spark plug. Magnetic fields made by coils direct the scanning beam to be accurate. That is where lines of resolution comes from. More lines = a better picture, Anyway if you coat the leaking carbon spot the coil will function better. If you can find a mica rock lift a tiny sliver of mica & cover the spot with the corona dope glue it over the bad place. Mica is great insulator & has a very high dielectric value! If you have a neon shop they will sell you a piece too. Bring a little bag to put it in.
Jeff

[tom-c]

I used a VOM and get no shorts to ground from either plug wire. I get the needle to move just a little between the two plug wires but it’s on the Rx100 scale. I think that’s ok. Right? Points are open with a match cover.
You guys provided a lot of info here so I have to take a little at a time!

[Author]

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