D7 Emergency ignition wiring. Can anyone help?

[Lesj]

Looking at a late D7 external coil wiring diagram, am I correct in assuming the following?
With the ignition switch in the normal position, negative battery power is permenantly supplied (via the switch) to one terminal of the coil (green wire) and the other terminal of the coil makes and breaks to positive earth via the contact breaker (maroon wire) which causes the coil to energise and create its spark.

However when the switch is in the emergency position, the battery power (brown wire) is removed and the green wire terminal of the coil is now permenantly connected to earth and the maroon wire terminal of the coil is connected (via the switch) to the dark red wire from one of the alternator coils.
This would appear to make the contact breakers redundant. Is the coil now energised by one side being permenantly earthed and the other side receiving power pusles from the alternator dark red wire in order to create a spark? If so, has the polarity of the coil terminals now swapped over whilst in emergency position? This is baffling me and any help would be appreciated.

[rhyatt]

I'll try to clarify.
Your 1st paragraph is correct , battery voltage via switch into coil , through coil to points , make and break , spark.
Can I assume you have a separate ht coil on the rear mudguard ?
Ok EMG position , AC voltage from generator coil goes via switch to coil , then as above.
This is why , the instructions were to get it running on EMG then switch it back to normal asap , to let the AC charge the battery and normality resumes.
If you run it lo get on AC you'll damage the points

[Lesj]

Thanks for getting back to me. I understand how replacing the ignition power supply with alternator power (Dark red wire) would work but I am still confused about the permanent ign coil connections.

According to the wiring diagram, in the emergency ign position, one side of the coil is connected directly to permanent earth (via the switch)
The other side of the coil is receiving a permanent power supply from the alternator (that same side of the coil is also connected to the contact breakers)
We know that the power supply to the ignition coil has to be broken in order to cause a spark.
How is the coil power supply ever interupted in this situation?
Opening the contact breakers wouldn't remove the permanent supply from the alternator to the coil.

[rhyatt]

hi,
a couple of preliminary questions -
engine number please?

According to the wiring diagram, in the emergency ign position, one side of the coil is connected directly to permanent earth (via the switch)
This cannot be correct surely - has a previous owner done work/modifications to your bike

have you got an external HT coil mounted on the rear mudguard ??
if so -
1) this coil will receive either 6 volts from the battery via the I position on the ignition switch in the headlight
or
2) AC via the E position on the ignition switch in the headlight

the POS side of the coil (maybe marked POS or +) goes to earth interupted via the contact points

their is a marvellous WIPAC publication that will give you clear diagrams for all the Bantam models

I cant seem to be able to send it to you - failing that your Haynes book or your owners book

[Lesj]

https://bsanotrun.angelfire.com/switchwireing.html

Hi, thanks for the reply.
This is a late GD7 engine (1965-66) with an external coil

In the Normal ignition position, the switch connects the battery (-) (brown wire) to one side of the coil (via the green wire) The other side of the coil makes and breaks to (+) earth (via the maroon wire and contact breakers) thus energising and de-energising the coil, (creating and collapsing the magnetic field) to cause the spark.
As we know, that's just a normal positive earth battery ignition set up.

However with the switch in the Emergency position, that green wire from one side of the coil is now connected straight to the frame earth (via the translucent wire) and the other side of the coil is connected to the dark red wire from the alternator (via the maroon wire) plus that same side of the coil is also still connected to the contact breakers.(The maroon wire runs from the switch to the coil and carries on to the Contact breaker) This also means the polarity of the coil terminals has swapped over but I don't think makes any difference.

With the engine running, this means the coil is constantly energised from the alternator but we know the coil has to be momentarily de-energised or disconnected in order to collapse the magnetic field and cause the spark.
Opening the contact breaker in this situation would not disconnect the alternator feed to the coil.

The only way I can see how power is momentarily removed from the coil in this situation is that when the contact breakers close, the power feed from the alternator power will take the path of least resistance and run straight to earth through the points and cease to energise the coil until the contact breaker opens again, when the coil would be re-energised.

That would mean that the spark occurs when the points actually close as opposed to when they open in normal running.

In this situation the ignition timing would be somewhat retarded and may explain why an engine sounds rough when running on emergency ignition.

[scifi]

I haven't got a Bantam so I can't help with the wiring diagrams, but I can confirm that the spark only occurs when the points open.

The spark is produced by an alternating voltage, the frequency of which is determined by the value of the coil Inductance L in mH, and the capacitor's capacitance C, in uF.   The C and L form a parallel circuit.

The reason the voltage is so high, is because the coil current (3 Amps.) is almost instantly stopped, and the energy in the coil is transferred back and fro to the capacitor.      Mathematically if the points open in 0.0001 seconds (allowing for arcing.) the voltage is 3A /0.0001 or 30 Kv.

[rhyatt]

The only bike engine I know that uses CLOSED points is a Villiers.
Either way, points breaking or points closing, the primary magnetism collapses thereby inducing a secondary voltage/spark

[Lesj]

I haven't got a Bantam so I can't help with the wiring diagrams, but I can confirm that the spark only occurs when the points open.

The spark is produced by an alternating voltage, the frequency of which is determined by the value of the coil Inductance L in mH, and the capacitor's capacitance C, in uF.   The C and L form a parallel circuit.

The reason the voltage is so high, is because the coil current (3 Amps.) is almost instantly stopped, and the energy in the coil is transferred back and fro to the capacitor.      Mathematically if the points open in 0.0001 seconds (allowing for arcing.) the voltage is 3A /0.0001 or 30 Kv.

Thanks for that but are you explaining here how the spark is created with an AC ignition coil - the type that sits inside the alternator and always receives AC power when the engine is running? That's the case with early Bantam ignition coils but the later bikes (like mine) have a conventional external coil mounted on the rear mudguard and with the switch in the normal running position, the ingition system is a straightforward positive earth DC battery set up whereby opening the points simply removes the power from one side of the coil and the spark is created. However when the switch is in the Emergency position the external ignition coil now only receives permanent AC from the alternator. Opening the points in this situation cannot remove power from the coil. The only way to break the feed to the coil and therefore create the spark would be to send that permanent AC directly to earth which would happen when the points close. I've posted a link to the relevant wiring diagram in an earlier post on this subject if you want to take a look.

[Lesj]

The only bike engine I know that uses CLOSED points is a Villiers.
Either way, points breaking or points closing, the primary magnetism collapses thereby inducing a secondary voltage/spark

Thanks, I wasn't aware of that.  This must be on an ignition system that shorts one side of the coil to earth when the points close as opposed to breaking the power feed to the coil when the the points open in conventional systems. Obviously in the Villiers set up the points cam would have to keep the points open for most of the cycle and only allow them to close exactly when the spark is required.

[rhyatt]

i can see I think what you are trying to understand , but what are you trying to do please?

without knowing the internal switching process of the ignition switch contacts when in the E position , it is difficult to answer accurately for you , I have 4 bantams , 3 x D7 with external coil on the nudguard and 1 D14 with a different WIPAC set up.

I have a wipac service manual if youd like me to send it to you , which will give you a different insight into the evolution of the Bantam wiring

[rhyatt]

i think our paths have just crossed - the coil needs a dwell period to "charge up" either by DC or AC and then a period when it all collapses.
the bantams went through an evolution of the cam angles during their life

[Lesj]

Thanks for the reply. All this began when I was studying a late D7 wiring diagram (external coil) and couldn't figure out from the wiring diagram how the coil de-energises in the emergency switch position. This link https://bsanotrun.angelfire.com/switchwireing.html shows the internal switch connections in various positions. (if you scroll down the page you will see the relevant diagram for the emergency ign position) In any situation, power has to be removed from one side of the ignition coil in order to collapse the magnetic field and create the spark. That's how coils work.
If you look at that wiring diagram, you can see that in the emergency switch position, the coil remains permanently connected on both sides (one side to earth and the other side to an alternator coil) The only way the coil could lose its power supply in this situation would be to take its feed (from the alternator) straight to earth. This would occur when the points close because in the emergency position that side of the coil is also connected to the points.

I would be very grateful for that Wipac manual if you could send it. Thanks.