Spark plugs firing every time the coil fires

[NickinSussex]

Hi there

I have a newly rebuilt GT6 mk3 with a 6V coil with a ballast resistor. An Accuspark unit is installed. When the starter is engaged the spark plug to #1 cylinder fires every time the coil fires (ie not once every 2 revolutions, but 3 times every revolution) - checked using the strobe tester. If I move the strobe tester to other ignition leads, the same problem occurs on the other cylinders. You can hear sparking inside the cap. When I took the distributor cap off , with spark plug #1 connected, the same thing happens  - you can actually see sparking from the central rotor contact in the cap to the #1 ignition contact. I replaced the cap with a new cap, and the same problem occurs.

The system has the standard feature where 12v is switched to the coil when the starter is engaged, but reverts to 6v when the starter is disengaged.  I removed the 12v connector on the starter solenoid and engaged the starter. This time the #1 spark plug sparked normally (ie once every 2 seconds), so it appears that when 12 volts is supplied to  the coil an immense amount of energy is being produced causing the "crossfire" inside the cap. With 6V everything is okay. 

I have been racking my brains as to how this could happen. The car ignition system was perfectly okay last year and the only thing I have done over the winter was rebuilding the carbs.

I could try replacing the coil, but I though it worth asking the community first about this problem before committing to the purchase.

Any ideas as to what could cause the problem would be appreciated. As an engineer I am flummoxed !

 

 

 

[johny]

Hi, I cant quite get my head round what youre explaining. Turning over on the starter motor with strobe connected to no1 spark plug you see the timing mark in 3 positions on the front pulley at 120º to each other and then once running theres only one mark showing at the correct position?

Then you say you 'took the distributor cap off and could see sparking from the central rotor contact to the #1 ignition contact' without the cap in place????

First check is to measure the resistance of the coil both low and high voltage sides and let us know the results.... 

[dougbgt6]

Nick,

Are you saying that with the rotor cap off you turn the engine and spark jumps from the centre of the rotor to the plug wires?!! If so something is not right with the coil and/or it's wiring. With nothing attached what resistance do you have across the coil? Should be 1.5 ohms for ballasted 3 ohms for non ballasted.

Doug

[Pete Lewis]

And double check the 12v ballast bypass is only feeding on cranking and not all the time  putting 12v into a 6v coil,doubles the HT  beyond  the insulation within  the dizzy

Dougs on  the ball with the coil resistance     ,

You may have a performance problem with the unit fitted

Pete

[johny]

As I understand it the problem occurs 'when the starter is engaged' so its almost as if the voltage isnt dropping enough during starting! Hence 12v+ are still being applied to the 6v coil even while the starter motor is operating. Have we got a 24v battery installed😵

[NickinSussex]

Thanks for your replies.

I am investigating the problem without trying to fire up the engine - just to confirm, when turning over with the starter the strobe light on the #1 ignition flashes at the same frequency as when I put the strobe light on the coil lead (ie 3 times per revolution). As I said, if I then take the cap off the spark leaps from the central rotor contact in the cap to the #1 ignition contact inside the cap - about 2 cm or so ! 

However, the sparking returns to normal when the 12v wire from the starter solenoid to the ballast resistor is removed ie when the coil is operating in its normal 6v mode.

I have checked the resistance across the coil high and low voltage sides - it is about 1.6 ohms and about 5.4 kilo ohms.   The ballast resistor also has 1.6 ohms resistance.

The battery produces 12v, the voltage dropping a volt or 2 while the starter is engaged.

I have tried an old set of ignition leads and still the problem exists. As I said previously, one of the first things I did was replace the distributor cap with a new one.

I am trying to get my head around the physics of the situation which suggests that much more than double the energy is being produced by the coil when the coil voltage is increased from 6v to 12v !

I guess the next thing will be to buy a replacement 6v coil and see if that cures the problem - there can't be many other things to try changing! 

Any more thoughts would be appreciated.

 

 

 

 

 

[johny]

I wonder if theres a problem with the rotor arm? You dont say its been replaced and if it had a very high resistance the spark may well take another route and track across the inside of the cap.....

[Anglefire]

Good call Johny, or cap. 

[johny]

Well I think Nick has already tried a new cap so theres only really the rotor and coil left to change. In theory shorted turns in the low voltage side of the coil could change the transformer ratio so giving a higher voltage on the spark side but those resistance readings seem to indicate that its healthy.....

[Anglefire]

He did, you are quite right! 

As you say though, it must be tracking somewhere.

Daft question @NickinSussex but have you tried disconnecting the ballast so it starts on the standard 6v coil? If the problem still persists it is properly weird, if not, it must be the extra voltage breaking down the rotor arm or cap - I know you have changed the cap, but it could be a faulty one.

As a thought, as its only plug one thats tripple firing (If I have read it right) can you fit the cap 180deg around (Moving the leads obviously  ) and see what changes - does it follow the cap or not?

[Pete Lewis]

Is this delco or lucas dizzy

Does the rotor have a rivet in the sweep plate ?

Lucas  copy rotors were notorious,  if lucas get a red one 

Is the coil only fed 12v when cranking ???   Not permanently 12v

Check there is no gap between rotor and, its cap  brush/contact.

Pete

[DanMi]

Don't forget modern repro parts can be bad out of the box, so just because the distributer cap is new doesn't mean its not the problem. I would try a known good cap and rotor arm if available.

 

[RogerH]

Hi Folks,

      if in doubt of your ignition parts I would recommend buying them from Martin Jay - The Distributor Doctor  http://www.distributordoctor.com/

I have no connection other than a happy customer.  Quality products.

 

Roger

[JohnD]

Nick describes seeing the sparking INSIDE the cap, when it is removed and the engine turned over, so that the charge gets across the insidee surface of the cap to the plug leads.  In this situation, where he can see it happening, the rotor arm is nowhere near the centre or plug ecletrodes, so the rotor is nothing to do with this problem.

I would have said that there must be a conductive surface inside the cap, leading to excessive 'tracking'.      If all the plugs fire at once, then this won't be a crack or a single track, but some layer of conductive substance.   I would suspect a worn central contact, usually made of carbon so that everything is covered in carbon dust.      BUT, he says that a new cap reprodcues the same fault, so that theory must be discounted.

Then, tracking can travel some distance acros the cap interior surface, but in the abscence of a conductive layer, or a crack, it would need one hell of a high voltgae to jump across an inch (?) of air.    I can only presume that the 6V coil is getting 12V all the time, so the HT ystem is getting about 30% more charge, and at least 40K volts, not the usual 30K.

John

[johny]

well Ive never tried turning my engine over with the dizzy cap off but maybe it too would track across the inside. However with a good rotor theres a much easier path to take so this never occurs in normal operation which why I suggested the possibility of a duff rotor....

 

[Nick Jones]

You shouldn't get that sort of behaviour even with the extra coil voltage......

Dodgy cap seems favourite.  New can be dodgy too or both may just be damp - WD40 is your friend.

The black rotor arms with the rivet are amazingly bad and consistently so.  A red rotor arm from the Dizzy Doctor or other reputable source is a must.

If the distributor drive gear is one or two teeth out if the ideal position it's possible that to get the right ignition timing you to end up needing to turn the main body to a position where the rotor arm doesn't point directly at any of the HT lead posts but lies between them.  This normally leads to cross-firing rather what is described but is something to check.

As mentioned above, sparks escaping in the wrong direction can be due to the right direction needing too much "pressure ".  Resistive leads in combination with resistive plugs and/or an overlarge plug gap might make the sparks look for other routes.

[Pete Lewis]

old school test for a rotor  is remove cap  hold king lead about 10mm from top of rotor

crank it over , if spark jumps to the rotor its got a short , if it doenst jump its fine 

do check the rotor sweep plate direction of rotation is as marked on it 

check with thumb on rotor that there is no side float on the centre spindle 

Pete

[johny]

with the power Nicks system seems to be putting out hed better be careful doing tests like that😂

[dougbgt6]

Although old school and not modern 'ealth and safety the spark will still take the shortest path to ground. Unlikely to be Nick, but he can wear his Marigolds if he wants.  And Pete is still with us. 

db

[johny]

with that spark going from cap centre post to the lead terminals I dont think marigolds are going to do so its got to be a pair of rubber North sea diving gloves🤗

[Pete Lewis]

Way back at work a real pain kept worrying about timing , taking the micky I said stick  lead in mouth and turn engine over you will find the sweet spot.

Next day Had to get security to  remove him from my office ,    you cant please some people

Pete

[dave.vitesse]

Back in the 1960's you were told too wear rubber gloves, rubber boots and keep one arm behind your back when dealing with high voltages.

The trouble these days is you would probably get arrested seen doing the above!

Anybody remember the following - "It's the Volts that Jolt. But it's the Current that Kills". It meant that a high voltage with a micro-amp current (minute) wouldn't kill. That's the theory we were all taught.

Back to track - I think the problem is with around double the voltage on the coil it produces a larger spark and this is finding faults in the ignition components. By the way non resistive plug leads don't help as they place or reflect greater energy on to the other ignition components and not absorb it. The leads become resonance and not only place a greater stress on the other ignition components, but also cause radio interference.

Dave   

[SpitFire6]

Hi,

 copper plug leads are a source of radio interference. They provide faster rise-times than resistive is my understanding.

Not heard before about resonance, reflected waves & causing of greater stress with copper leads fitted.

Please explain.

Cheers,

Iain.

[dave.vitesse]

Two areas 1/ Not all the power is absorbed in the combustion chamber, some is reflected back. Hence the resistive leads absorb this energy. 2/ The spark at the plugs excites the leads which act as an aerial, the lead length has a resonance frequency. Basically due to the spark there is a high AC content. i.e. A spark transmitter. This can increase the energy on the other ignition components. As a result any weakness may well show up.

Dave

[SpitFire6]

Hi Dave,

 Thanks for the explanation. Resonance would be > 100MHz?

Fit another D_Cap.

Cheers,

Iain.