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Bosch DU 4/2 - - intermittent spark on one circuit

Pete LaBelle

Registered
Last Subscription Date
11/15/2013
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
 

stufforbud

Subscriber
Last Subscription Date
09/17/2019
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
 

J.B. Castagnos

Registered
Last Subscription Date
01/01/2006
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.
 

Andrew Mackey

Moderator
Last Subscription Date
05/14/2017
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.
 

Scotty 2

Registered
You might have a look at the rotor and carbon brush, where the high voltage comes off the rotor. .
Also check the other LT carbon brush. People always forget this one and it's just as important as the HT one. I think it may be in the same position as the picture below. It's under the tension strap. You can see the bonded wire that's screwed onto the base. The brush should move very freely and a lot of times that don't.
Cheer Scott

 

J.B. Castagnos

Registered
Last Subscription Date
01/01/2006
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.
 

Scotty 2

Registered
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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