spark at points and plugs??

[bloodthirstysystem]

with the engine running i am getting consistent sparks between the points on my right cylinder but not for the left side. the left side only has a visible spark every few rotations. should i see a spark there?

both plugs get a spark, timing is spot on, point gap is good, points cleaned with lacquer thinner. coils are 6ohms primary, 17kohms secondary.

at idle it's fine but when accelerating it feels like it's missing sparks. bike lurches and misses..

carbs just cleaned, didn't fix it.

 

[pamcopete]

blood.........,

Install new condensers.

 

[bloodthirstysystem]

i'll try it. can i use anything that i can buy at radioshack or autozone for condensers? if not i'll order from mikes, just don't wanna wait a week and pay $10 for shipping

 

[pamcopete]

blood.......,

Go to Advance Auto. Buy 2 of their #G120P @ $8 ea, which is a condenser for a 1967 Camaro.

Might do cheaper elsewhere. AutoZone #U1511 @ $6.99 is the same thing.

 

[gggGary]

+1 Strong spark at the points is bad, condensers are cheap and a fairly common failure. there are different sizes and ratings of condensers but any good one is better than a bad one. quick check? Swap condenser leads, see if the spark follows the condenser.

 

[bloodthirstysystem]

awesome, thanks pete. i will try switching the leads tonight. i'm sure it's bad, bike has been sitting since about 1985 or so, probably the original part.

should be any spark at all between points? weak spark good, blue spark bad??

my plugs don't have a very blue spark, they work but not "fat and blue". would the condenser also cause this? coils seem to read within spec

 

[gggGary]

an occasional weak spark at the points is fairly normal but I have to admit I am a bit rusty on points ignition. Just got a refresher working the kinks out of an old CSR305. The stock coils were not that spiffy even when new. Unlike wine, 30 years of aging doesn't help. There is a reason for the popularity of aftermarket HP coils.

 

[bloodthirstysystem]

do condensers need to be grounded to the frame? i understand they hold the back emf temporarily but does it ground out or get sent back through the wire


i got a set yesterday and made an aluminum bracket which is bolted on where the coils used to be.

 

[pamcopete]

blood.......,

Yes, the condensers need to be grounded.

 

[Sundie]

that's not actually the main job of the condensers. They allow the (relatively) slow ability of the charging system to provide voltage to provide a quick charge to the coils. They're just capacitors in line with the coils. Points close, ground/charge the coils, then open, allowing the coils to discharge, and the collapsing magnetic field through the secondary windings grounds out through the spark plug. The condensers allow a full charge to build in the coils. You can technically run without them, but the result will be a weak spark, and none at all at higher rpms.

 

[Sundie]

oh, and sparks at your points is generated as they close. Nothing to worry about, unless they start pitting from age.

 

[pamcopete]

Sundie,

Well, close....Basically, the condenser provides a path for the voltage created by the collapsing primary field in order to complete the circuit and produce a current which is necessary to induce a voltage in the secondary. In fact, the combination of the coil and the condenser creates a series tuned circuit that "rings" at the resonate frequency created by the coil and condenser when the points open, so the resulting secondary voltage is a very brief AC excursion.

Without the condenser, the relatively high voltage created in the primary by the collapsing field (about 500 Volts) will jump the very small gap of the points as they open, but not the larger gap of the points when they are fully open, so the result is a very brief flow of current across the gap of the points as they open, which does induce a relatively low voltage in the secondary. As the engine speed increases, this initial very small gap is short lived, thus limiting the secondary voltage.

When the points close, they bounce. The initial contact will start the current from the battery flowing in the coil, but a few milliseconds later, the points bounce open and the small accumulated "charge" in the coil will collapse and produce about 300 volts across the points, which are almost closed, so the voltage is enough to jump the gap. Then the points close again, and bounce open again, so yet another small spark is created until finally the points stay closed and the coil can fully charge up.

Bouncing points also shorten the effective dwell angle. If the points are bouncing for, say, 5 degrees, then the dwell angle is effectively reduced from 93 degrees to 88 degrees. This phenomena is most acute at higher RPMs because there is less time to charge up the coil and a shortened dwell angle reduces the available time even further, resulting in high speed miss which cannot be curred by just cleaning the points.

When the points open, there is the initial creation of voltage in the primary by the collapsing field so something similar happens and there will be a small spark as this initial voltage jumps the gap of the almost closed points. Less bounce occurs with points opening of course, but there is some flexing of the stationary point that attempts to follow the moving point as it opens and it will "catch up" to it and reestablish contact, so there could be several sparks as the points open as well due to contact bounce, but not as severe as closing points.

Higher quality points are designed to minimize contact bounce. This is done with a stiffer spring and a more solid mounting for the stationary point and a reinforced arm for the moving point. The bounce in really poor quality points can be so bad that a spark in the spark plug is created when the points close due to extreme contact bounce. This means that a spark is created in a very advanced timing position, about 180 degrees before it is supposed to, like during the intake stroke.

Even the best points wear out in a variety of ways. The mounting for the stationary point weakens and starts to flex more, leading to more bounce when the points open. The spring weakens as well which leads to more bounce when the points close. This can be seen as more obvious sparks coming from the points.

So, take a look at spark of your points when they are new. If the sparks get worse, and cleaning the contacts does not help, the points are done for and need to be replaced. And don't buy cheap points.

A perfect set of points would open and close instantly and no spark would be created. That perfect set of points is called a transistor.

 

[Sundie]

Sundie,

Well, close....Basically, the condenser provides a path for the voltage created by the collapsing primary field in order to complete the circuit and produce a current which is necessary to induce a voltage in the secondary.

I sit corrected on that one. Must have been the heat at lunch.

In fact, the combination of the coil and the condenser creates a series tuned circuit that "rings" at the resonate frequency created by the coil and condenser when the points open, so the resulting secondary voltage is a very brief AC excursion.

Well... No. There's no A/C involved here. Unless you're counting the collapse of the magnetic field as 90 degrees of the sine wave, which it isn't, any more than a simple square wave 0-5v is A/C (though it has a/c-like characteristics, and you can often use it as such)

But the field collapse is entirely independent of everything at that point - the fact that it's collapsing through the secondary winding is what causes the spark at the plug. What's happening in the primary winding is of no concern to the event causing the spark at the plug.

I concede and agree with everything after this.

A perfect set of points would open and close instantly and no spark would be created. That perfect set of points is called a transistor.

Well, there *is* a *slight* lag, but it's so high that it's effectively not a concern. So not perfect, but close enough for disco.

 

[breahn riley]

Pete just dropped some knowledge.

 

[pamcopete]

sundie,


If there was no current flow in the primary when the points open, you would simply have a secondary voltage that is a product of the collapsing field acting directly on the secondary.

What actually happens, is that the collapsing magnetic field induces a much higher voltage in the primary that 12 Volts, but that would be wasted unless you had some device to close the circuit in the primary to produce current from that higher voltage.

Typically, the primary voltage will go as high as 400 Volts when the magnetic field collapses. When that voltage goes through the complete circuit that includes the coil and the condenser, it produces a much larger magnetic field than just the 12 volts would, so the resultant secondary voltage is orders of magnitude higher.

I can see this phenomena on the scope in the lab. If I just use a transistor that opens the circuit to the coil, no spark results because there is no current flowing in the primary. There is a secondary voltage produced by the collapsing magnetic field, but it is not a high enough voltage to jump the gap of the spark plug in the test setup.

If I use a transistor with a clamping diode from collector to ground, the 400 + volts induced in the primary cause the clamping diode to conduct, producing current in the primary, which produces a magnetic field greater than what is produced by the 12 Volts alone. The actual transistor in the PAMCO has a built in clamping diode that turns the transistor on to create the complete circuit for the higher induced primary voltage.

In a points system, the condenser serves the same purpose. When the points open, the condenser / coil combination will ring briefly at their resonant frequency. This "ringing" is literally transformed to the secondary where it produces a much higher voltage.

You even said that the coil will not produce as high a voltage without the condenser. That's because without the condenser there is no current in the primary because there is no path and the resultant voltage induced in the secondary is insufficient to jump the spark plug gap. The only reason there is any spark at all is because as the points open, they transition through an infinitely small gap that does briefly provide a path for the primary current in the absence of the condenser.

Here is an excerpt from Wikipedia that says basiscally the same thing:

When the contact breaker closes, it allows a current from the battery to build up in the primary winding of the ignition coil. (The current does not flow instantly because of the inductance of the coil.) Once the current has built up to its full level, the contact breaker opens. Since it has a capacitor connected across it, the primary winding and the capacitor form a tuned circuit, and as the stored energy oscillates between the inductor formed by the coil and the capacitor, the changing magnetic field in the core of the coil induces a much larger voltage in the secondary of the coil.

 

[Sundie]

so, your explanation would seem to suggest that it's not a single spark, but a series of microsparks then?

 

[bloodthirstysystem]

thanks for all the good info dudes

i used two condensers from a 1965 ford at $5 each. i'm still seeing some small sparks at the points but now it's both sets instead of just one. they're also not as bright or as frequent as they were before. the bike runs perfect

 

[pamcopete]

sundie,

The coil is essentially a step up transformer, and as such, it is a low pass filter. Whatever happens on the primary side is distorted by the time you see it on the secondary because the high frequency components do not pass from the primary to the secondary.

The nice little but brief oscillation on the primary will have its higher frequencies removed, so what you see on the secondary will not be an exact copy of the primary.

The other thing that happens is that the induced energy from the primary effectively boosts the voltage on the secondary to the point that there is sufficient voltage to jump the spark plug gap. Once this happens, there is current flow in the secondary and now the collapsing magnetic field can induce power directly into the secondary. Once the spark gap has been bridged, it can be sustained with a much lower voltage.

Transistor ignitions are easier to understand because they do not use a condenser to create primary current. They are closer to your concept of a simple square wave on the primary producing a larger square wave on the secondary due to the step up action of the coil windings.

However, a few realities do creep in. There is a certain amount of built in capacitance in the coil itself created by the windings and the space between the windings. This capacitance is much smaller than the capacitance of the external condenser of a points system, so ringing does occur, but it is incidental and of a much higher frequency. Also, it is not essential that there be any ringing as there is in a points system.

And, the coil itself with it's built in inductance and capacitance is still a low pass filter.

The primary signal starts as a much purer square wave created by the clamping diode or transistor conducting when the primary voltage reaches a certain voltage, typically 400 Volts, after the transistor has been turned off. But. like any square wave, the sharp vertical leading edge of the square wave is made up of an infinite mixture of frequencies, none of which will get past the low pass filter effect of the coil, so only the primary frequency gets past to the secondary and lacking the shaping effect of the higher frequencies, it essentially looks like a few cycles of a sine wave of relatively low frequency with some steady state persistence due to the collapsing magnetic field acting directly on the secondary.

 

[Sundie]

Absorbed and filed. Thanks Pete!

 

[ironwill]

I'm going to be checking my coil's tomorrow. The manual I read says 3.9 primary, 8 secondary, +/-10 ohms. blood.....you showed 6 primary, and 17 secondary, assuming I read this all correctly. Does that mean the 3.9 and 8 are minimums, and anything over that is good?