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Well-known member
Well, I rowed in today after a failure to start. I have a O470 GPU. I pulled a plug wire and....no spark. Could this be the magneto? If so, how do you test it? What else could it be and how can I troubleshoot it?
First open the inspection plate, it will have 4 small screws in front, if ita a Bendix mag. in there you can see the points, check the gap, shold be arround 16 thousands or so,,,,make sure the contacts are clean....if still no spark, you need to get the mag rebuilt, could be a number of things.....coil, condencer, etc,,,,,make sure you know how to time the engine, or pull put the mag correctly.
Check the mag switch first. I have had many folks wanting a mag overhaul-- that a switch fixed. Run a spad lug or the equal between the mag and the swith, just disconnect thr spad to check the switch.
I thought the mag switch killed the mag when shut off. If disconnected, switch dies, broken wire, etc. occurs your mag should be hot constantly and is a dangerous situation. Or so I thought. AP guys talk about broken wires something to beware of with hot mags on plane, because if you turn the prop, the cylinder could fire.

A quick way is just disconnect all spark plug wires. Pull top plugs and stick a plug in each of the wires and turn her over. You should see the blue/white spark. If no spark, you have a problem.

I have seen lots of condensors in the mag have the wire break off the end. It is the little cylinder (1.5" long X 0.5" diameter) under the top cap Davie said to look under. It is easy to replace and can be ordered from Chief Aircraft online.
A switch can fail to open or closed position ( on or off) If the switch is closed the mag will not fire--groundeed.
Un-plug the mag (no wires going to the mag) this means the might is hot, turn the boat over, if it fires, your switch is bad, if not could be points, condensers, or the mag may be wet, or need re-built..if the boat fires with the mag un-plugged, when going to turn it off, ground the wire to the mag to shut it off.
I took off the inspection plate and noticed that the wire to the condenser was cut because it was run along the top and over the shaft that spins when you crank it. I'm not sure what that part is called. Anyway, I think it was causing a short. Additionally, the wire was barely hanging on where it was soldered to the condenser itself. I took it off to re-solder it and I'll put it back on tomorrow. Anybody ever hear of anything like this?
I forgot to say that I bypassed the switch and tried to crank it to make sure that wasn't the problem. I think the switch is OK.
BoHawg - I don't know why, but it seems like about 5 years ago, we had problems with mags breaking the condensor wire fairly regularly. Sometimes the solder job works and sometimes not. If you take the part number off the side of the condensor, you can order them off Chief Aircraft for about 12-15 bucks.

When the motor got rebuilt, I got the mags checked and worked over as well. Not a problem since, so I still have 2 spare condensors in the cabinet.

Hope it fixes the issue.