Recently, I posted an afterfire problem with my '67 327/300. It was thought and on good advice, that it was most likely a burned exhaust valve or broken exhaust valve spring. While trying to think it out, I referred to an old Motor's Manual diagnostic and the last possible cause cited was "failed condenser". I decided to replace the condenser first(read: easiest to do) and lo and behold, no more afterfire! I then cleaned all 8 fouled plugs and it now runs fine.
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Re: Exhaust "Afterfire" Solved
Interesting! You should probly check you points then because they more than likely took some abuse for a while. I am wondering what the mechanics of it all were? Hmmmm, would the bad condenser cause the plugs to fowl which resulted in various deposits on the plugs that remained hot and caused a preignition/poping? Or, caused a shorter dwell time that allowed the spart to initiate sooner. I would recheck all your timing and dwell. Later, Terry -
Re: Exhaust "Afterfire" Solved
With a 'failed' condensor in the distributor (either due to lost capacity or from excessive internal leakage of the capacitor), the effective dwell of the spark would be curtailed, approaching a single impulse vs. a timed duration. You would expect rather incomplete combustion, poor fuel economy and, yes, a carbon coating on the plugs' ceramic insulators as a result.
This is why testing capacitors requires two separate checks: (1) absolute charge storage verification, and (2) leakage or a dissipation factor check. Think of a capacitor as a storage vessel for electrical charge--similar to a bucket for storing water....
One factor is its storage capacity, so many micro farads for electrical charge in a capacitor or so many pints/quarts/gallons in a bucket. Now, if you have a bona fide bucket rated for, say, 1 gallon of liquid storage but the bottom is litered with holes, it's not much of a bucket!
Yep, it DOES hold 1-gallon, but it leaks like a sieve!!! The same goes for capacitors. You can put a cap rated at 0.5 mF on a tester and verify it really stores 0.5 mF, but the part is absolute JUNK unless you also measure it's leakage or dissipation profile....
The dissipation factor of a cap, how much charge does it RETAIN when the charging voltage is removed for a given fixed period of time is of equal importance. This speaks to the integrity profile of the capacitor--does the bucket have holes in it? If so, how badly is it riddled with holes?
Dissipation factor ranges from 0-100 indicating how much of the cap's rated storage capacity is lost in a give time interval once charging current is removed. Typically, electrolytic caps like those used in the distributor or for radio frequency 'hush up' purposes, are 'good' when their dissipation factor is in the 1-5 range (1% to 5% of stored charge is lost within a short time period).
So, anyone who sells you a capacitor without quoting BOTH the cap's measured storage profile (uf) AND it's measured dissipation factor (%) could well be providing you a 'bucket' riddled with holes! The dissipation factor of a given capacitor climbs like a skyrocket when the dielectric insulating the cap's two storage plates is defective (break down or 'punch through') and such can/does happen over time and as a function of electrical overstress....
Once that happens (excessive internal leakage), the capacitor (archane term from the days of Farraday is condensor), the cap is essentially junk! Caps with HIGH leakage or a large dissipation factor can look like an electrical short circuit and do more harm than good.Comment
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Re: Exhaust "Afterfire" Solved
Thanks for the explanation. I am self taught when it comes to how an engine works and especially the ignition system. So I am probably screwed up somewhere along the line. I look at the ignition system as having a low voltage side and a high voltage side. The capacitor sees the low voltage side of the coil in regards to building the field in the coil. The capacitors purpose is to protect the points during opening and closing to prevent arching across the points. Effectively, the capacitor/condensor provides and temporary location (bucket) to store electricity for a very short period of time. Though I have not thought it through completely, it may only be during opening that the capacitor is actually working to protect the points (what you think). But, you also mention that the capacitor effects the duration of the spark that is seen at the plug coming from the high voltage side of coil. I had not considered that before. I assumed it was simply a characteristic of the coil and how it was made. But, It sort of makes sense. Are you saying that the high voltage side is firing the whole time the points are open, not just a portion of that time? Things are happening pretty fast when an engine is running. I just figured the high voltge side just fired untill the field colapsed (or reached a voltage that it could no longer jump the gap on the plug). Can you elaborate on this. Thanks, TerryComment
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Re: Exhaust "Afterfire" Solved
Your understanding of the Kettering ignition system is a bit flawed.... There's the primary side of the coil and the condensor (capacitor) inside the distributor connected to a voltage supply source through the ignition points (a switch with some ohmic contact). Therefore, you have two energy storage devices (the coil's primary stores energy in the form of magnetism and the condensor stores electrical current in the form of charge). This forms an RLC filter whose impulse response (transfer function) will ressonate at a given frequency.
When the switching function occurs (points change state from closed to open), the 'filter' is 'hit' with a state change impulse and the filter 'rings' at its ressonant frequency. Essentially, the coil and and condensor play 'catch' the electrical energy they've stored and this causes the spark to to persist or 'dwell' until the stored energy on the primary side is effectively dissipated by the natural resistance of the primary side elements as well as energy being 'sucked out' of the circuit by transfer across the coil's transformer core to the secondary side of the coil and on to the spark plug....
If the primary side condensor is 'bad' in some fashion (wrong specific capacity or leaking internally), the circuit will 'ring' at wrong/incorrect frequency changing the persistance profile of the spark at the plug. Does this make more sense?Comment
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Re: Exhaust "Afterfire" Solved
I was told that if your condensor was "bad", that the car would not start. I have been chasing a low idle miss for a few weeks now, and a condensor is about the only thing I havn't tried yet. My car is a 71 SB, auto, AC car. This problem has just appeared in the last few weeks and the only time in the eight years that I owned it.
ChasmanComment
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Re: Exhaust "Afterfire" Solved
Question...is the high voltage side of the coil constantly firing? Or, does it only fire with the opening and closing of the points? In which case it should only fire when the rotor comes close to the post in the distributor cap.
Your explanation is very good, I will have to read over a few times to soak it in. The idea of playing electrical catch seems to make sence to me. But, I need to read it closer. The idea of the system ringing (osilating??) is something I have not thought about. I understand that a magnetic field is built into the coil by the low voltage side and when that feild colapses (points close?) it is coupled to the high voltage out put from the coil.
I will read your responce some more. Thanks for the conversation about it. Always wanted to know how. TerryComment
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Re: Exhaust "Afterfire" Solved
Understand, from the discussion I gave on capacitors, 'bad' has all shades of grey to it.... It's not a one/zero situation. If the cap is internally leaking and not holding charge as it was designed to, expect the effective dwell to collapse in lockstep to the degree of leakage. If the cap is DEAD SHORT, then yep, she's bad and you've essentially shorted the coil's primary thus eliminating spark altogether.
But, there's no need to guess. A competent mechanic with an engine analyzer can put your car's ignition waveforms up on the scope and actually SEE what's going on!Comment
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