1975 - 4.9 is firing badly. Distributor cap, or anything else..?

I think a return to the basics is in order. I would start by bench testing the spark plug wires with an ohm meter. The problem might be something as simple as a bad crimp connection in the wire between the coil and the cap. Then connect the coil wire to a spark plug and have someone crank the engine while you watch the spark. You should have a consistent strong spark. That would tell you that the breakerless ignition and Bosch amplifier are working properly. If the spark is weak or erratic then you are much closer to determining the problem. The rest of the ignition system is the rotor and cap which I doubt are the problem since you already replaced them.

Ivan

 

This Youtube isn't about ignition in the way you need but it does show how you can turn your laptop into an oscilloscope for not much money which I think will allow you to test everything. I think I'd start with making sure that pickup in the distributor is generating the right wave form and amplitude to drive the Bosch Box. I bet you can record the results in a file and send it to the guy who was so helpful checking out your box. He might even suggest one to buy and give you some pointers. I wish these had been around when I had all my trouble back in 1992. I wasted two years ...



Good luck.

 

You mentioned the coil before, but did you replace it?

Also you mentioned not getting any spark, did you check the earths?

 

Please note that an ignition coil failure can be permanent, causing a no spark no start condition or it can be intermittent, causing a cylinder-specific misfire condition or a random misfire.

 

If #3 is consistently missing that is a good thing as it makes it easier to find the problem.
Do you have the original style Cavis solid wire spark plug wires or are they resistive wires?

Swap the #3 wire at the spark plug with either the #2 or #4 wire, do the same at the cap. Is #3 now firing? If it fires then the problem is in the spark plug wires.

If it is still not firing then put everything back the way you found it and check continuity from inside the distributor cap for#3 to the spark plug connector . If the car has solid wires there should barely be any resistance.
If that all checks out then it is time to move upstream a bit and check the breakerless ignition. If I recall the Bora distributor uses metal rods that rotate in front of a magnetic sensor. This generates signals that tells the Bosch amplifier to trigger the coil. Rotate the engine until the rotor is pointing to the #3 wire on the cap. Is the gap between the metal rod and the sensor consistent with all the other cylinders? Perhaps the sensor gap for some reason is too large. Find out the proper gap setting and make sure it is within specs.

Did you try what I suggested of connecting a spark plug to the wire from the coil to the cap? If there is problem in the breakerless ignition it will manifest itself with a spark pattern that is not consistent. If spark pattern is not consistent the problem could also be in the coil. If the spark is strong and consistent then the problem has to be between the cap and the spark plugs.

Ivan

 

OK---let me rephrase:
IMHO---FWIW---please spend 90 seconds and try a new coil.

Regards.

 

I see that parts swap out mode is still the preferred approach.

Get a better tool and learn about how to diagnose the entire ignition system with it. Once you do this starting at the very lowest level of the system through the final HE output you'll find it.

Maybe post up a video with good audio ot the engine running poorly too.

 

Art, electrical gremlins are always fun to track down... the reference I made to the earths, or grounds are that there should be some grounding straps or cables between various components in the engine bay, ie battery and engine, to the chassis etc. Whenever troubleshooting electrical faults I like to check that the connections are sound and haven’t come loose or broken from vibration etc. Any loss of earthing in the ignition system can reduce electrical continuity in the system and cause faults. Mostly they are easy to check and cheap/easy to fix if you know where to find them.

good luck with the search

 

Step 1
Disconnect your engine's main negative battery cable.

Step 2
Pull the main ignition coil wire, the one that leads to the distributor, off the coil. Disconnect the small grounding wire, attached to the side of the coil.

Step 3
Turn on the multimeter and set it to the "ohms" function.

Step 4
Insert one of the multimeter's probes into the center opening of the coil, contacting the metal terminal inside the coil. Touch the second probe of the meter to the ignition coil's grounding terminal. The meter should read 6,000 to 15,000 ohms. If it does not, the coil's secondary winding is faulty.

Remove meter's probe from the center terminal and touch it to the terminal bolt on the opposite side of the coil from the probe touching the grounding terminal. The meter should read between approximately 0.4 and 2 ohms. If it does not, the coil's primary winding is faulty.

 

IMHO---supporting data for my 'telepathic' diagnosis

 

There is absolutely nothing wrong with those coil readings as they match those on my Bora. That does not mean that the coil could be breaking down under electrical load but at least those reading are perfectly fine.

Art: please follow the suggestions I gave you and do not deviate. It seems to me that you are randomly attacking this problem and that is not the way to go at it. Start at the spark plugs and work backwards towards the Bosch amplifier. Forget about mickey mouse tests like checking for sparks in the dark Use your ohm meter to check continuity in all spark plug wires and especially the wire between the coil and the cap. Then check for the strength and frequency of the sparks as they exist the coil by connecting a spark plug to the wire from the coil to the cap. Do this and then report back.
I built a spark plug with a wire and an alligator clip to make sure the plug is properly grounded while doing this type of test.

Ivan

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Good to hear Art !!!!

Mike