Electronic Ignition

[AlanBartlett]

Now I know this is probably blasfame but of late during some fairly longish journeys I seem to experience some sort of electrical mis fire, which result in me either replacing coil/condensors/points multiple times, more times than id like to really. I dont know if jowetts are common for this or its the poor quality componets Id like to say mainly intermotor equivelants or lucas parts. But I was thinking on or wondering about fitting some kind electronic device if its possible to cure possible failures. Or would it not be possible? Not really sure on this subject. Perhaps more experienced members could advise.

[Tony Fearn]

during some fairly longish journeys I seem to experience some sort of electrical mis fire

Hello Alan,

Can't help with the electronic ignition query, but with standard equipment, I didn't have any trouble with the 'Flying Fox' on its way to the Crieff, Bideford or Llandrindod Rallies, the first two of which were well over 700 miles round trip.

On one of the rallies prior to these, a certain VSCC/JCC member, in his 'cabriolet' was stopped at the roadside during the scenic run with similar problems, with numerous later-model-owners in attendance all scratching their heads.

I suggested coil to dizzy wire problems, and we found that there were only two strands of the wire at the dizzy end from the coil holding up. The rest of the strands had broken.

We replaced the wire and JB went on his way.

Also, check all of the earthing points in the car, perhaps undoing and cleaning them with emery paper berfore lightly greasing them before re-assembly.

I'm sure that you don't really need to replace the "coil, condenser and points multiple times".

The answer will be much more simple ---- perhaps!!

Tony.

[Forumadmin]

Make sure the coil is wired the correct way around and that is a coil of the correct polarity. One way to check is with a graphite pencil placed in the arc from a spark lead. The ions should flow to the lead not the block if negative earth, I think....must check. The orange flame goes from tip of pencil to lead .

[AlanBartlett]

during some fairly longish journeys I seem to experience some sort of electrical mis fire

Hello Alan,

Can't help with the electronic ignition query, but with standard equipment, I didn't have any trouble with the 'Flying Fox' on its way to the Crieff, Bideford or Llandrindod Rallies, the first two of which were well over 700 miles round trip.

On one of the rallies prior to these, a certain VSCC/JCC member, in his 'cabriolet' was stopped at the roadside during the scenic run with similar problems, with numerous later-model-owners in attendance all scratching their heads.

I suggested coil to dizzy wire problems, and we found that there were only two strands of the wire at the dizzy end from the coil holding up. The rest of the strands had broken.

We replaced the wire and JB went on his way.

Also, check all of the earthing points in the car, perhaps undoing and cleaning them with emery paper berfore lightly greasing them before re-assembly.

I'm sure that you don't really need to replace the "coil, condenser and points multiple times".

The answer will be much more simple ---- perhaps!!

Tony.

See now thats the thing, it can only be something simple. Its quite odd, when its under load youll get a pop pop misfire. once every so often. I also noticed on the way up to shepton mallet, it would misfire, and its like you lose all power, but are still running, and then it seems to cycle back up again. Now to my memory sounds alot like coil. Though it only happened a few times during the trip. And on the way back, it persisted until I got to the last big hill before getting back home, I had to stop, changed the coil. for a new one id bought at the show in preparation. It got me up the hill but sill popped and backfired a bit with this on. So id like to say it isn't the coil this time. So ill be switching it back to the older one and running around a bit more to try and figure it out.

Something else Id noticed on the way up that the points had seemed to close up and i thought this would have been the source, so reset them, but still popped and backfired. about once every while. The leads from the ignition to the coil are ok, all with insulated ring crimp terminals (Id just did these a few weeks ago) So am happy with the supply. My only other thought is the condensor or even a plug.

[george garside]

A very common cause of misfire ( after loose and faulty wiring) is tracking within the distributor head or body . This can be caused by a slight crack in the cap or by a minute accumulation of oily vapour and dust on the baseplate forming a path for electricity to try to reroute itself. In the days when everything used points, condensors etc I used to clean inside the distributor once a month with petrol and replace points at around 6000 miles ( not just on Jowetts but any make)

As Tony said points & condensors should not need replacing frequently particularly on the llow mileage now done on old cars.

As to fitting electronic ignition it can be done but the simplest arraangemet ( which may not be available now) used the points to switch the current. The points however lasted much longer as they were only passing a low curent. There are firms that advertise in the classic car mags that will appareently do one of set ups for classic bikes and cars. My own feeling is that all it does is reduce the need for regualr points adjustment and replacemnt but has no advaanatage performance wise over a properly adjusted and mainatained traditional system

george

[Tony Fearn]

Alan.
Before this started happening, had you been fiddling and fettling somewhere under the bonnet?
To add to what George suggests, have you tried another dizzy, or cap or even another rotor arm? If the rotor arm is slack on the fitting at the top of the drive shaft, pack it with a little bit of paper to stiffen it up.
Is the timing as it should be - see the instruction book.
Is the dizzy drive-shaft very worn in the top bearing? This will vary the points gap as it rocks around.
If bearing ok, are you using 18 thou as a points gap?
Is the carburettor bowl clean of deposits?
Remove and have a look at the carb jets and their seating washers whilst checking the bowl.
Do you have a modern type petrol filter in line?
Is the petrol pump working ok?
Are the plugs in good order with no cracks in the insulator?
Are the carb nuts tight on the manifold?

[AlanBartlett]

On further inspection of the points, Ive found that they were black as your hat. And one of the contacts had come loose within its housing. So perhaps this could have been the trouble. But having done about 300 miles with this set up I still wonder if its component related. Ive tried the gap setting of 18 thou on the points now (though the book recommends both 10 and 12 any reason behind 18?), and taken it for test run up and down a fairly longish hill, the performance has improved, but the tick over is quite lumpy and rough. I shall go through the list youve put up Tony and report back the results.

[Forumadmin]

There is some misconception about ignition coil polarity and how important it may be (or not). For most people it is only a matter of lack of knowledge, perhaps followed by getting some bad information. How the coil works (with points and condenser) to create a high voltage spark is covered in another article. This article is a discussion about electrical polarity of the coil and spark plugs.

The ignition coil is essentially a low voltage to high voltage transformer with about 100 to 1 ratio of windings and voltage. The coil case is not grounded, and both primary and secondary windings inside are "floating" or isolated from the case. The only thing the windings have in common is one end connected to the same primary terminal, and it really doesn't matter much which one. Being a transformer it must have pulsating or alternating current to work. Initial pulsating is done by connecting and disconnecting the primary circuit ground connection. Alternating current then comes into the function in a big way by electrical "ringing" in the condenser at very high frequency. A transformer is not affected by polarity, since it is an alternating current device, so it matters not to the transformer what the input or output polarity may be. Any polarity on the primary side and any polarity on high tension side will produce the same quality of spark.

cp0.gif

Why then do we worry about coil polarity? Because the spark plugs do care which way the electrons are flowing in the high tension circuit. The spark plug has a thermally insulated centre electrode (surrounded by ceramic). With engine running the centre electrode runs substantially hotter than the exposed end electrode. Design of the ceramic insulator determines how hot the centre electrode will run, leading to the designation of hotter or colder spark plugs. As electrons go, they love to jump away from a hot surface and fly toward a colder surface, so it is easier to drive them from hot to cold rather than from cold to hot. End result is a difference of 15 to 30 per cent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going. So the spark plug prefers to see a voltage potential that is negative on the centre electrode and positive on the end electrode for the very first hop of the spark. Oddly enough, this has nothing to do with polarity of the vehicle electrical system. The common connection inside the ignition coil and the relative direction of winding of the primary and secondary coils on the armature define the polarity of the output on the secondary due the direction of the induced currents between primary and secondary. Think of physics at school and imagine how currents flowing through a wire cause a magnetic field, and then if that magnetic field changes how this induces a current in a wire in that magnetic field. When the current through the primary is broken by the points, the collapsing magnetic field creates a voltage/current in the secondary. It also creates a back electro motive force or voltage on the primary which the capacitor captures to stop arcing across the points. The small amount of charge stored in the capacitor does continue to die away through repeated reflections through the primary causing a ringing or continuation of the spark via the secondary. Note the black arcing of the points suggests that the capacitor was not doing its job and thus the spark was far from good as the back emf was leaking through the arc across the points.
The common knowledge bit about electrons is that they carry a negative charge. For electrical bits (similar to magnetic bits) opposites attract each other and negatives repel. This means the direction of flow of electrons in a car is from the battery negative post through the wiring to the battery positive post (not necessarily intuitive). If you reverse cable connections on the battery the current flows in the opposite direction through the vehicle wiring. For most original functions on the JOWETT this matters not one whit to anything, as most original equipment in the JOWETT is not polarity sensitive (except maybe the optional radio). As one end of the primary winding in the ignition coil is connected to one end of the secondary winding, reversing polarity of the coil primary side will reverse the drive direction of the spark current on the output side (even though current in the vehicle low voltage wiring still flows the same way).

So reversing vehicle electrical system polarity will reverse direction of spark drive. The engine still runs either way, but spark might be more reliable under marginal conditions if you get it right. The simple fix for this is to reverse the two primary wire connections on the ignition coil. Because the output spark is very much higher voltage (20,000v) than the car battery (12v), it doesn't care if the battery polarity is helping or hindering by a meagre 12 to 14 volts in battery potential. It is probable that with the wrong current direction across the spark plug you will get poor starting and premature failure of the ignition coil as it has to work harder. It might also cause failure of distributor cap and plug leads as higher voltages build up to start the spark. It is not a good idea to disconnect a spark plug lead and have the ignition coil fire as the spark will find its way to ground somewhere, either inside the coil, or from an HT lead to somewhere sensitive to high voltage (you), or in the distributor cap to cause breakdown of the insulation.
So how do we know which way to connect the ignition coil for best results? Original production coils were generally marked on the primary terminals "SW" for Switch and "CB" for Contact Breaker. This was assuming the vehicle wiring was connected for positive earth (positive battery cable grounded on the chassis). If you reverse battery polarity (going to negative earth), then these coils need to be connected with "CB" to the ignition switch and "SW" to the distributor points wire. Later issue ignition coils are marked "+" and "-" on the primary terminals. These are more no-brainers, as you only need to match the terminal markings to the battery posts. For positive ground the "+" terminal goes to the distributor (to be grounded on the engine block). For negative ground the "-" terminal goes to the distributor (to be grounded on the engine block).

cp1.gif

If you are still sceptical about all this, there is a quick way to check directly which way the current is flowing in the high tension circuit. Disconnect a spark wire from a spark plug (or the coil wire from the distributor cap). Hold this HT wire near a grounding point (or near the connector end of a spark plug), and position the tip of a graphite pencil in between. When you crank the engine (no need to start or run) you can observe the resulting spark jump between wire and pencil, and between pencil and ground (or spark plug). A flare (hard to see) toward the plug (or ground) shows correct polarity while a flare toward the coil shows reversed polarity. If the flare goes toward the coil, just switch the primary wires on the coil and make note of the connections for future reference.

cp2.gif

Can't see the flare? Not sure yet? You can also check spark polarity using an analog (moving needle) volt meter. Hook up a voltmeter with the negative lead to the plug terminal and the positive lead to the block. Set the meter on the highest volt range. Crank the engine over (no need to start it), and you should see an upward swing of the voltmeter needle (don't be concerned with taking a reading). If the needle swings down off the scale, your coil is hooked up wrong. To correct it reverse coil primary leads. Do not worry about the coil markings, but make note of them for future reference.

I am currently investigating why two attempts at using the Accuspark electronic ignition system have failed causing the units to go short circuit. I suspect it is because an overvoltage condition exists on what was the contact breaker lead caused either by the back emf or a spark jumping out of the HT lead to the CB(-ve) terminal (my SA is negative on chassis). I now have a new coil supplied by Accuspark and new electronic contact breaker, so we will see what happens. But I am doing that after the trip to Shrewsbury.

[Forumadmin]

Further to the previous post an explanation into the action of the coil is appropriate.
Some of the postings on the MGA and other forums from which much of this was taken are not really correct as the writer does not use the concept of current correctly. For instance, current does not flow through a capacitor. An electrical charge is stored in a capacitor which in the case of its use across the contact breaker points stores the back electro motive force caused by the breakdown of the field in the coil when the points open. Also the voltage produced in the coil is not generated by the current but by the change in magnetic field within the coil which is described by Faraday's Law. This back emf raises the voltage on the now open points and the capacitor to about 300 volts as can be seen here. Note the back emf would create the high voltage without the capacitor there due to the inductance of the primary coil, provided the points do not arc and dissipate it. The capacitor reduces the instantaneous voltage across the points.
This circuit explains the physics better, although the primary and secondary are not connected in quite the same way.

cp4.jpg

This back emf charge is stored by the capacitor and leaks back through the primary causing a current that in turn creates magnetic field. The capacitor and inductance of the two circuits form a rapidly decaying oscillation. Once the arc is formed, additional voltage spikes caused by the oscillation of the rebuilding and collapsing field, will continue until the voltage is not great enough to maintain it. This is where some knowledge gained when fixing my TIG welder is being replayed! But I am no expert!

Note the spike is a negative voltage. The coil current takes some time to build up due to the inductance of the coil when the points close again.

This next picture shows the oscilloscope trace that can show if all cylinders are operating well. If any cylinders are poor then the high voltage peaks will be different. Weak mixtures, bad leads or dizzie cap will all show up.

cp5.jpg

[Forumadmin]

I did this tonight on the Jup just to refresh my memory. I first started the engine, as trying to do it with a cold engine or cranking by hand is not effective. Having stopped the engine I removed the lead to the spark plug and put a suitable bolt into the cap and positioned its end (using a cloth to steady it) about 1.5cm from the top of the spark plug. Then I placed the tip of a pencil, also steadied with a cloth, so its tip was halfway between the two metal points. I turned the lights out and using a torch started the engine , and then observed the spark. Note I did not hold the pencil as that may give you a sizeable shock! You also need to make sure the other end of the pencil is away from any metal as the spark will conduct down the graphite. Doing it this way the engine runs evenly on all four cylinders and you have lots of chances to see the difference between the spark to the pencil in each direction. You might not observe the slight difference in colour of the sparks but you should see the spark to the plug flares out as it touches the spark plug.