[BryanH]

Last night I replaced the coil with a previous one and had no change in the way the car ran. I also tried wiggling the wires, and that did nothing.

Then I put my timing light on each plug wire as suggested by engineer, and all looked good except cylinder number 8, which was very sporadic. It looked like it was not firing more than it was firing. Then I put the light on the coil high-voltage wire, and it looked like there was a miss there too. Then I swapped the cylinder 8 and 6 plug wires to see if the plug wire was bad, but 6 stayed good and the miss stayed on 8.

The miss on cylinder 8 made me think the distributor cam might have a bad lobe, but the fact that I could see a miss "upstream" from there at the high-voltage wire makes me wonder. Is it possible for faults to cause bad readings "upstream"? Could a bad spark plug in no. 8 cause the bad readings I saw at both the plug wire and high-voltage wire?

Any help appreciated.

Thanks,
Bryan

[stu]

If the high voltage path was shorting out, would that cause the secondary winding to fail over time? Maybe that is a clue. Also, is the coil ground OK? It relies on the case making a solid connection to the block, right? (No separate ground lead)

[george kujanski]

Well, if your observations of cyl.8 are correct, you have determined the following: the coils themselves don't seem to be going bad; cylinder 8 is the problem; the plug wire for cyl 8 is not the problem.

Ok, so the possibilities are: a fault in the cap for #8, (carbon track,etc.) bad plug, plug gap too wide, ( forces the coil to develop higher voltage for that plug which either it cannot do, or the voltage developed is leaking off somewhere else since the plug is not firing. Example: say the bad plug needs 20kV to fire, the coil develops 18kV which is too much for the insulation on the wire, boot, or cap. The spark will jump or leak off at the weak point and never jump at the plug.

So the spark energy may be leasking off elsewhere, such as the boot to close to the head, etc.

Check those things, check the gap, temporarily replace the plug with another one, see if the problems stays or moves to another cylinder, you get the idea. Remember, the above is based on the assumption that your observations of #8 are valid.

Good hunting, Regards, George

Hey, on your idea of a bad lobe, what about a bad distributor gear, also? Could cause a bad dwell variation at one point in the rotation.....

[BryanH]

I forgot to mention that I replaced the distributor cap with a previous one too, which did not help.

What about the miss that I could see at the high-voltage wire? Could that be caused by a bad plug or distributor or anything else "downstream"?


Thanks,
Bryan

[george kujanski]

Well, if you actually see a miss in the coil HV wire, that indicates that the problem could be in the point system, i.e., the points themselves, the lobe, the gear, etc., Since the opening of the points is what actually causes the coil to fire. Anything that affects the instant of time the the points open will cause the coil to misfire. since you say it is one cylinder only. Also, the dwell being too short, (points not closed long enuf) will cause a misfire at the coil output.

Another way you would see a misfire is if the coil is actually developing voltage ok, but the plug gap is too wide for the spark to jump it AND the wires, etc are in good shape insulation-wise so that the spark does not jump anywhere else. Most inductive-pickup timing lights are triggered by the spark CURRENT in the wire. if there is no spark CURRENT,i.e. (no spark discharge) then the light may not flash, the cause being a possible bad plug.

As before, good hunting. Regards, Georg

[BryanH]

Thanks for all the help! I have learned a lot from this post.

[george kujanski]

Stu: I believe there is no internal ground connection in the coil. The case is not connected electrically to the coil inside.

Regards, George

[BryanH]

If a distributor lobe is worn, is it possible to compensate for that by setting the dwell longer than spec?

Thanks,
Bryan

[stu]

So where does the secondary coil return to? Just curious, I always assumed it was tied to the case but maybe it returns thru the battery feed.
All the ideas seem like they would making a contstant misfire, wonder why changing coils without doing anything else fixes it? Strange.

[george kujanski]

Stu: it's strange, all right. All problems can usually be explained, however. The HV return is thru the battery feed. The coil schematic shows the opposite end of the secondary side of the coil connected to the (+) terminal.

Bryan: if the lobe is worn, meaning the high spot is closer to the base diameter, your dwell, at least for that cylinder, is already increased. The problem in that case is that the points barely open, if at all.

Dwell adjustment is a compromise. On the one hand you want max dwell (more time closed) to build up the field in the coil, especially at higher revs. You also want the points to open as wide as possible to minimize the arc when they open. These are conflicting requirements, however, so the compromise is about 30 degrees, which gives an open gap of about .018, as I recall.

I also find it hard to believe that the lobe would only be worn in the #8 position. More likely the gear or a lot of play in the shaft bushing, perhaps.

Regards, George

[engineer]

So the problem appears on 8, then look at that plug, the no. 8 terminal on the top and inside the cap, the rotor contacts 8 okay? Looks okay, then look at dist. cam lobe in no. 8 position? flat, shiny, etc. compared to the other sides? Dist. shaft tight up and down, no rocking side to side? good ground to dist. plate under cap? etc. Your problem has to be downstream of coil.

[Sun Tuned]

How about the ignition switch?

[BryanH]

Well, I replaced the no. 8 plug and the car runs OK now. Not as good as it could, but I suspect I need to change the other plugs as well. Plus I will check the distributor, etc. as described above.

It still seems strange that replacing the coil made the car run good for a while. Maybe it will remain a mystery.

Thanks everyone who helped me with this. I learned a lot.

Bryan

[Sun Tuned]

Oops! Definitely the wrong post. 10 new switches wouldn't fix a bad cylinder. Well that's what I get for trying to eat, chase the child, and not pay attention. Glad you got it fixed.