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Help me understand the points circuit...
Help me understand the points circuit...
I am working on re-assembling the magneto body on my R50/2. In addition to the open coil, I am also changing the points, condenser, and felt. I am replacing the OG coil with an Emerald Island one...with the second ground wire.
It's this second ground wire that has me confused.
My understanding is that the little bolt that fastens the coil primary ground, condenser, and movable point spring needs to be electrically isolated from the points mounting plate. I can test this with my multi-meter. With one lead on the bolt and the other on the points plate, there should be continuity with the points closed and it should go OL (open) when the points open.
My re-assembly works this way until I add the coil primary ground connection to the bolt. With this connection, there is always continuity between the bolt and points plate regardless of whether the points are open or closed. This makes sense, to me, given that the second ground wire from the coil is fastened to the coil clamp which is connected to a pole shoe, which is connected to the base plate, which is connected to the brass plate which is connected to the points plate. So when the points are closed, the circuit is complete through the points and when they are open, the circuit is complete through the coil. (I understand it's not quite that simple.)
If this wiring is correct, how does the opening of the points cause the field to collapse and create the high voltage spike? If my hook up is incorrect, what am I missing? Am I correct that when I open the points, there should be no continuity on a "bench wired" magneto body?
Many thanks for any information that you can share.
Best regards!
Jim
- wa1nca
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Re: Help me understand the points circuit...
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- bmw electrical system .pdf
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54 R51/3, 55 R50/Velorex 560 sidecar, 64 R27, 68 R69US, 75 R75/6
Ashfield, Ma
USA
Re: Help me understand the points circuit...
I had found that document and agree that it is invaluable. It doesn’t give me enough information to know if I should be concerned that I still have continuity between the coil primary at the points bolt and the points plate with points open.
Thanks for helping me increase my knowledge. I very much appreciate it.
Best regards!
Jim
- wa1nca
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Re: Help me understand the points circuit...
54 R51/3, 55 R50/Velorex 560 sidecar, 64 R27, 68 R69US, 75 R75/6
Ashfield, Ma
USA
Re: Help me understand the points circuit...
Dave
- vechorik1373
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Re: How the magneto WORKS and problems
The BMW uses the wasted spark method. While both plugs fire at the same time, only only cylinder is on the compression stroke the other is on the exhaust stroke. On the cylinder that is on the exhaust stroke, the spark is wasted, nothing happens, the one on the compression stroke ignites the fuel and air.
The coil is actually a siamese coil in the sense that it has ONE primary winding, and TWO secondary windings, one for each spark plug. If it did not, have two secondary windings, it would only fire the plug that had the least resistance of the two.
And remember, the faster a magneto turns, the STRONGER the spark gets. There are 10,000 turns of wire in each secondary winding. The enamel on the copper wires is the insulation.. What kills the original coils is that it gets quite warm under the front cover. Copper expands with heat. The constant cycling of heating up and cooling off, eventually will chafe the enamel insulation off the wire, causing a partial short out of the windings when hot. That is why when cold, the coil will work. When it gets hot, it shorts, but the speed of the magneto is great enough (because the faster it goes the stronger it gets) the engine will keep running. But if you shut it off, and try to restart it, there will be no spark at all any longer. When it gets cold, it will start again.
The magneto is turning very slow at kick start speed. You can't kick it as fast as it turns even at idle. That is why not only do you have to have a good coil and a good condenser, but you want to do everything you can to improve the spark at kick start speeds.
The mag body is made of dissimilar metals. Over time, those different metals will corrode where they are in contact with each other. The corrosion gives you a HIGH RESISTANCE ground, that will destroy condensers, and lessen the strength of the spark a kick start speeds. When the condenser fails, you will no longer have a spark. (always carry a spare condenser with you when riding,l just in case) So it it is important to take the mag body apart, and clean the surfaces that touch each other with sandpaper or glass bead all of the parts clean. And use a little dab of corrosive inhibiting grease (like NO-OX-ID grease) on the surfaces that touch each other. This is the best stuff to use.
In order to get the max spark and kick start speeds the magnet rotor must be timed correctly to the body.
The procedure is:
1. Set the flywheel so the line above the S mark is in the window dead even with the "notch" in the viewing window.
2. Notice the scribed line on the face of the rotating magnet? That is a timing mark. Notice the "V' notch in the top center of the brass face plate of the body? When you install the body on the engine, rotate the body left or right until the scribed line is dead in the middle of that V notch, then tighten the two 6 mm nuts and never move that body ever again!
3.Timing of the engine is accomplished by rotating the breaker plate the points are mounted on, in combination with the point gap. You must rotate the breaker plate so when the points crack open (when it fires) it will be when the line above the letter "S" comes up even with stationary mark in the window. If, when rotating the breaker plate, you run out of travel, and the points don't crack at the S mark, then you must go back and change the point gap. Remember, widening the point gap makes the engine fire earlier (more advanced) narrowing the point gap makes it fire later. (more retarded.
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Re: Help me understand the points circuit...
Technical Adviser, Former owner, Bench Mark Works
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- schrader7032
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Re: Help me understand the points circuit...
This page describes the magneto ignition system:
https://en.wikipedia.org/wiki/Ignition_magneto
The condenser (aka capacitor) seems to perform a dual function, one as a "snubber" to protect the points, but also as an energy storage with the energy going back and forth to the coil. Not being that aware of this type of electronics, hard to grasp that the small condenser can hold all of the energy for the plugs.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
- wa1nca
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Re: Help me understand the points circuit...
Yes
The coil wire has to be removed if using a ohm meter to see when the points open and close
It is a PITA to remove the point wire from the points connection
See a coil that Max BMW sells
You can modify your new Emerald Island coil by cutting off the point wire on the coil, striping and tinning it, and then connect it to the screw terminal that has the wire going up to the ignition switch board to disable the spark when the key is up or removed
Then when you need to remove the point wire to check when points open just unscrew it from the terminal connection so your ohm meter will read when points open and close
Thanks Vech for your post
54 R51/3, 55 R50/Velorex 560 sidecar, 64 R27, 68 R69US, 75 R75/6
Ashfield, Ma
USA
Re: Help me understand the points circuit...
I have a basic understanding of how the magneto works, but I’m still puzzled at seeing continuity with a DVOM with the points open and closed.
It doesn’t appear to be problem, though, as when I hook up my magneto tester...not the DVOM... it shows the points working properly. I picked up that tool because I could tell that having to disconnect the condenser and coil ground to set static timing would be a bit of a hassle.
Again, thank you so much for helping me expand my knowledge.
Best regards!
Jim