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mjlyam

Montesa cota 247 ignition

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Hi all. Losing my mind here... the points on my cota 247 don't seem to have any effect unless I disconnect the two wires screwed to them. With these wires.. black and condenser.. removed I can check moment of opening with a multimeter beep. Replacing the wires and I get no break in the beep as the points open!! Is this right? Doesn't seem to make sense. I am running the black to the coil/condenser under the tank and the pale pink to earth. Green and yellow I discard as they are lights etc and not required for trials. Points are new... coils not shorting out... correct resistance in generator coils.....what have I misunderstood??? 

This is a rebuild of a box of bits. 

Help please.... Mike

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This might seem to be a mad question but you are wired to the ignition generator coil and not the lighting coil....madness over. You should be able to put a PP9 battery wired to the contacts to either show a break in the circuit when the contacts open using a torch bulb or a multi meter set to either continuity as you have already done or set to 20v to show on/off when contacts open. With the correct wiring connected as soon as the contacts begin to open you should see the multi meter show no continuity. When you get the sort of results you have it usually means the generator ignition coil is faulty. 

You should have one wire from the contact breaker insulated side going to the HT (high tension) coil and the condenser connected in series with this wire. The other wire from the LT (low tension) should be earthed. I’m sure Montesa use a black wire for the live, although not so sure about the earth so I would find a wiring diagram to confirm the pink wire is correct. Keep us posted.

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Mike,

If you P.M. me your direct e mail address, I can send you a scan of the wiring diagram with colours, if it helps :)

 

 

Montesa wiring diagram Cota 247.jpg

Edited by majesty
managed to add file to post

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I pulled all my lighting circuits off years ago but Pink to earth does not sound correct.  The circuit is earthed by the points connection to the magneto base

 

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Mike - 

The reason you are having to remove the c/b. points wires to show when the points open is because you are using a continuity meter/ohm meter.

In a flywheel magneto such as you have the condenser and h.t. coil are connected in parallel to the  l.t. coil windings, which means that regardless of whether the contacts are open or closed, you will still have continuity. (Effectively, the contacts are always grounded........) 

On 11/7/2018 at 3:50 PM, section swept said:

You should be able to put a PP9 battery wired to the contacts to either show a break in the circuit when the contacts open using a torch bulb or a multi meter set to either continuity as you have already done or set to 20v to show on/off when contacts open. With the correct wiring connected as soon as the contacts begin to open you should see the multi meter show no continuity.  

You should have one wire from the contact breaker insulated side going to the HT (high tension) coil and the condenser connected in series with this wire. The other wire from the LT (low tension) should be earthed. I’m sure Montesa use a black wire for the live, although not so sure about the earth so I would find a wiring diagram to confirm the pink wire is correct. Keep us posted.

This advice would apply to coil and battery (D.C.) ignition NOT flywheel magneto (A.C.) ignition in discussion here.

On 11/7/2018 at 3:50 PM, section swept said:

 When you get the sort of results you have it usually means the generator ignition coil is faulty. 

Clearly, no it doesn't......

To time your magneto the accepted method is to use a timing "Buzz Box" which will indicate when the points open using a change in resistance, rather than an actual break in continuity. Additionally, if you are using a d.t.i. or timing stick/plunger in the spark plug hole, it's an awful lot easier to hear the opening point than trying to watch the flywheel/contact points/piston position indication, etc. all at the same time ! 

Here's a link that you may find helpful :  http://www.dansmc.com/buzzbox.htm

HTH - let us know how you get on.

 

Edited by lorenzo
adding active link
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Mike - 

Any progress ? Did you visit the "buzzbox" site ?

I just turned up this:- , 

flywheel mag. schematic 001.jpg

which may well explain better what I was trying to convey in my last post.  

You are of course free to ignore any of the above and/or form your own opinion and act accordingly - it's your bike,etc........

Edited by lorenzo

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1 hour ago, lorenzo said:

Mike - 

Any progress ? Did you visit the "buzzbox" site ?

I just turned up this:- , 

flywheel mag. schematic 001.jpg

which may well explain better what I was trying to convey in my last post.  

You are of course free to ignore any of the above and/or form your own opinion and act accordingly - it's your bike,etc........

When the contacts open the current ceases to flow, the condenser (acts as a damper) prevents excess sparking and arcing at the contact faces and helps with the back emf. As the current to the coil has been stopped by the now open contacts the magnetic field generated in the HT coil collapses and creates the HT spark at the plug via the HT lead. The Buzz box is just the same as using a multi meter set to continuity...the bleep or buzz...indicating current flow or a complete circuit, the bleep cease when the contacts just start to open indicating an open circuit..no current flow. Of course the contact base is earthed but the moving contact is insulated, thus when contacts are closed a circuit is made, when open the circuit is broken. 

Testing the resistance through the points (across the contacts faces) indicates the condition of the points, high resistance to current flow  = poor contact condition therefore a low resistance would be indicated on a new or refaced set of contacts. By your reasoning with the contacts always earthed they could not act as an on/off switch which is what their function is. This would mean the HT coil would never be able to create a high tension spark at the plug because it is always in the charge condition. You need to break the circuit to enable the magnetic field generated to collapse!

Edited by section swept

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12 hours ago, section swept said:

When the contacts open the current ceases to flow, the condenser (acts as a damper) prevents excess sparking and arcing at the contact faces and helps with the back emf. As the current to the coil has been stopped by the now open contacts the magnetic field generated in the HT coil collapses and creates the HT spark at the plug via the HT lead.

In a d.c. (battery and coil) ignition system indeed it would......... Unfortunately, a flywheel mag. ignition doesn't work like that  - for the reasons already stated.

12 hours ago, section swept said:

The Buzz box is just the same as using a multi meter set to continuity...the bleep or buzz...indicating current flow or a complete circuit, the bleep cease when the contacts just start to open indicating an open circuit..no current flow.

The buzz box is NOT just the same as using a multimeter, as the bleep or buzz will continue to sound, but change note - the whole reason for using it is that when the contacts start to open there is still a complete circuit. - look at the circuit schematic again, in particular the ignition coil. You will see that there are two coils (l.t. and h.t.) either side of the core.

12 hours ago, section swept said:

Testing the resistance through the points (across the contacts faces) indicates the condition of the points, high resistance to current flow  = poor contact condition therefore a low resistance would be indicated on a new or refaced set of contacts

That's quite correct..............But that's not what a buzzbox does. We are not testing resistance or points condition, but indicating a change in resistance  in the circuit when the points open.

12 hours ago, section swept said:

 with the contacts always earthed they could not act as an on/off switch which is what their function is. This would mean the HT coil would never be able to create a high tension spark at the plug because it is always in the charge condition. You need to break the circuit to enable the magnetic field generated to collapse!

That's your reasoning, but once again you are describing the operation of a d.c. (battery and coil) ignition.

12 hours ago, section swept said:

By your reasoning, etc. 

 I'd love to be clever enough to claim it as my reasoning - I'm not.........and it isn't. 

If you don't believe an a.c.mag. flywheel  ignition system could possibly work you have failed to grasp the basic principles. I admit I struggled with it myself before getting my own buzz box

I'm rapidly running out of ideas to convince you........ 

Edited by lorenzo
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