theory of ignition - troubleshooting

[HalcyonS]

distributor/coil discoveries

Sooo....

I worked over the entire ignition system. Plugs good. oily, but not sooty. Gaps good. Checked and lubricated distributor centrifugal springs, changed out points, condenser and rotor.

How do you know if a condenser is shot?

Rotor was fairly worn - see comparison pics of old and new ones. You might find them amusing.

The contacts in the cap seem deeply grooved too. (see pic). I have a replacement cap but it seems cheap, with aluminium contacts instead of brass - should I use it?

Tested resistance on coil. 7.1 ohms. Should I worry about this?

The good news is, it started. The bad news is:
1. it suddenly died after 5 min. (Then it ran for 20 min)
2. it won't idle and dies at below 1500 rpm when warm, 1750 when cold.
I'm assuming this is a gap/ timing issue. I set the gap to .018.
An tips of rough and ready timing without a timing light?
thx!
Simon

[ndutton]

Tested resistance on coil. 7.1 ohms. Should I worry about this?

7.1Ω? That number is so out of whack it doesn't make sense. Maybe we need to revisit your measuring procedure, something's really wrong.

[HalcyonS]

> Maybe we need to revisit your measuring procedure, something's really wrong.

I have a good meter. I tested with wires attached and detached. Several times. No variation. And I tested a spare, (afaik new) black prestolite coil with same meter, same method, same time, same location : 4.3 ohm.

I guess I'll just whip the old one off and fit the new one next time I'm at the boat.

[ndutton]

I agree Simon. Your testing method is spot on so who are we to deny the results? The 4.3Ω coil is in the range we expect.

[ashleyco]

You can have ignition on, have distributor cap off.

[hanleyclifford]

About that condenser question; can anyone answer? Is there a resistance specification for the condenser?

[hanleyclifford]

You really need to test dwell with a meter; I'll bet dollars to donuts that .018" gap is not giving you sufficient dwell.

[joe_db]

A discharged capacitor will show very low resistance at first and then as it charges the resistance will seem to get very high. There are meters that specifically test capacitors as well and I bet some automotive meters have a condensor test function.
Also look here:

http://www.smokstak.com/forum/showthread.php?t=21175

About that condenser question; can anyone answer? Is there a resistance specification for the the condenser?

[JOHN COOKSON]

To Recap

Sooo....
How do you know if a condenser is shot?
Simon

Condensers can fail in two ways:

Internal short between the inner lead and the outer can. If there is such a short the engine will niether start nor run because the points will always be grounded even when open.

Lack of "capacity". The condenser doesn't adsorb the electrical pulse from the coil well. The points will show excessive pitting. You can always but a fancy tester if concerned but IMO it's easier just to change it out.


TRUE GRIT

[HalcyonS]

I appreciate you guys working this through with me. I'm the kind who likes (to think he) understands a situation rather than just having a procedure to fix it.

The coil/condenser/points/rotor/sparkplug is a fascinating bit of analog power electronics. Its a bit Rube Goldberg. Everything hangs together because of timing and resonances (ie matching the coil and condenser).

So I'm good with all this, the only remaining mystery is why my coil is 7.1 ohms. On some post I read, someone made a smart observation, that most A4s have been tinkered-with by folks who *thought* they knew what they were doing, for years or decades. I don't know how a coil fails, but I think there's a good chance that the wrong one was fitted. But its been running fine the 7 years I've owned the boat. So maybe coil value isn't so critical, at least for points systems. I'm torn now between the 'if it ain't broke don't fix it' camp, and the fixit camp.

[ndutton]

Your understanding will extend to other aspects too like the effect on the coil of roughly twice the dwell with electronic ignition compared to points and the similarity of coil function and starter solenoid behavior explaining why Facet recommends against using the R terminal bypass in the fuel pump circuit.

It's all related.

[JOHN COOKSON]

Halcyon
You will run cleaner spark plugs if you get more "pop" or higher voltage out of the coil.
You could run a new coil with 3.5 to 4 ohm reistance and keep the 7.1 ohm coil as a back up.
BTW: How does the arc across the spark plugs look? Blue-white or sort of dark yellow? The 7.1 ohm coil may be working but only marginally so.
Just my $0.02 worth.

TRUE GRIT

[Dave Neptune]

An ole' one

Simon, that may be one of the old 8V coils from the ole days. They were the ones that required the ballast resistor and the use of the terminal on the starter. They were fed the 12v for a hotter spark at start and once started the ballast dropped the voltage for the 8v coil. I still run into one of those once in a while.
Did your coil bracket on the engine have a set of mounts on it or even the ole ceramic insulated resistor still in place? It mounted right on the side with 2 screws.

Dave Neptune

[joe_db]

Are you sure you don't mean 6 volt coils?
AFAIK the ballast resistor was invented when cars went to 12 volts and they wanted to use the same coils.
8 volt stuff was - AFAIK - an aftermarket upgrade to 6 volt cars that would barely start when cold. You could get 8 volt batteries, coils, lights, etc. back in the day.

BTW - To study more on the theory of ignition, it is interesting to compare and contrast "electronic points" like the Pertronix that store energy in the coil and units like the MSD systems that store energy in capacitors.

[Dave Neptune]

Nope 8

Joe, no I mean 8v. Many of the old A-4's like mine were like that from the factory. You had to be careful not to buy a 6v as you would have a weak spark using the supplied ballast. Lots of the early engines auto, duty rated and marine engines were set up like this when manufacturers went from the 6v standard to a 12v standard. The reasoning was to lower the voltage passing across the points.

Dave Neptune