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Magnetos, Ignition Coils and Spark Plugs Discussion about magnetos, buzz coils, spark plugs, ignitors and low tension coils. |
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Bosch DU 4/2 - - intermittent spark on one circuitthis thread has 7 replies and has been viewed 478 times
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#1
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Hey guys -
The Bosch DU4/2 magneto on my Rumely had some spark issues. Removed it from the tractor and cleaned the points, removed the distributor block & cleaned that and the 'spark rod' that that comes thru the center of the mag. Have attached a stiff wire to each of the 2 spark plug wire terminals and bent them around to the housing with about a 3/16 to 1/4" gap. Made an attachment for my cordless drill to drive the thing to test. I'm getting a good blue spark on both circuits, but the second one that fires in the sequence only fires about 50% of the time. This is true for running with the impulse tripping or faster than impulse speed. What should I be looking at that might be causing the second terminal to only fire intermittently ? Thanx ! Pete |
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#2
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Sometimes for odd reasons the cams on the contact ring are shimmed to different heights and that causes the points to open a different amount and at a slightly different time. It is possible one set of points is not opening at the "sweet spot" of armature rotation to get the best spark.
Bud |
#3
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1/4" is a large gap for an old mag, you're stressing the insulation. If it fires 3/16" I would set the plug gaps .020 -025" and see how it does, the same coil is firing both plugs, look for leaks-cracks opens in cap, rotor, wires.
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#4
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If the coils are good, 1/4" should not be a problem. I would look for carbon tracks or corrosion on the distributer cal and rotor. internal corrosion on the cap terminals increases resistance. You might have a look at the rotor and carbon brush, where the high voltage comes off the rotor. there might be a bit of carbon dust in the housing that would divert spark.
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#5
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Weak magnets are also a possibility.
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info@mcdonaldcarb.com |
#6
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Cheer Scott |
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#7
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It was common for the magnetos to have a built in safety gap, this was a gap with a path to ground to prevent the mag from destroying itself if a plug wire came off while running. You may be at the point where the fire is jumping the safety gap at times. Close your gap a little and try it, you may not have a problem.
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#8
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Hello all
All of the replies could lead to intermittent spark and worth checking. The safety gap is a far gap if this video is anything to go on. I love old magnetos. It seems to me that the older they are, the better they are. This one was found in the bottom of a bucket of rubbish at an auction. It's one that must of been made around the time of WWI. The Bosch had been machined out and Liberty stamped in. All I did to this one was a good clean and service and that was that. Cheers Scott |
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