47 V8 ignition mystery

[Brian]

I don't know if this has any relevance, and I only heard about it through a mate, an old friend, very clued up with flatheads, I respect anything he says, so take it for what it's worth.... His experience was with a new adapter plate for early distributor, the top mounted plate that replaces the original coil. He told me the centre carbon brush was made of plastic!! I know there's a lot of shonky repop parts out there, Craig, is your centre contact in your cap actually carbon?

 

[Old Henry]

Craig, is your centre contact in your cap actually carbon?

The center contact has the appearance of brass around the edge with a black domed insert that has the appearance of carbon. See photo. There is no resistance from the wire end to that brass part but 30 ohms resistance at the "carbon" center.

Does it matter? I hate to just "try the new cap" because it is such a chore to do so - reinstall the whole wire loom plus the generator and fan. I'd really rather get a definitive knowledgeable answer to whether that resistance in the "carbon" center matters.

 

[Brian]

Does it matter?

Well, it did to my mate; he couldn't get spark to transmit through plastic!

 

[TerryOH]

The center terminal is carbon it touches the rotor and conducts the high voltage, from the coil to the rotor. To assist getting the plug wires all the way into the cap terminals add a very small amount of dielectric grease to a cotton swab and rub it in the cap, spark plug wire terminals. Check all nine of the plug wire terminals for conductivity about O. Ohms for the plugs. From the plug end of the spark plug wire to the inside of the distributor cap. Do this just before installing the cap to the distributor.

 

[jimTN]

The distributor point plate plunger was mentioned, have had problems with them in the past, removed it and ran a wire to the points. Also, the "brush" in your ign switch could also need cleaning. both of these items tend to cause intermittent problems or problems after put under a load.

 

[FlatheadTed]

As a Experiment I once strung some welding wire across the the three bolt Diver and hung a HT wire off the coil down to the Rotor about 8mm gap ,the car started and ran fine ,,this could work on the road to get you home ,

 

[Old Henry]

Any other ideas? I'm at my wits end. I've never been so stumped by anything on this car in my life.

I had very high hopes that replacing the distributor cap would solve the problem since some had suggested that engraving the terminal numbers on the inside of the cap (which I've done for17 years) was the problem. It wasn't. Just like every other time I did something I thought would fix it, I hooked up the wiring harness but didn't install the generator or fan and took a short drive up a steep hill. All cylinders fired just fine. Then, after installing the generator and fan, I took a longer ride and the same old problem came back - cylinders #3 & 6 not firing consistently. I pulled the wires off the plugs and held them by a head bolt to see if they'd fire right not going through the plugs in case the plugs were the problem. The spark to the head bolts was erratic the same as through the plugs.

 

[Old Henry]

As a Experiment I once strung some welding wire across the the three bolt Diver and hung a HT wire off the coil down to the Rotor about 8mm gap ,the car started and ran fine ,,this could work on the road to get you home ,

Fortunately, my problem is not bad enough to stop forward motion to get home. It just makes getting home, or even away from home, less fun because of the chronic hesitation.

 

[FlatheadTed]

My post is in answer to 67 the spark will jump ok ,Could you swap the offending wire to the one next to it and see if the problem moves ,Have you changed the coil yet ??

 

[Old Henry]

My post is in answer to 67 the spark will jump ok ,Could you swap the offending wire to the one next to it and see if the problem moves ,Have you changed the coil yet ??

Switching wires is, indeed, all there is left to do. Unfortunately, I don't have the crab cap that would make that so easy. I have the stock rabbit ears cap the means changing one wire means changing them all as far as I know. If I don't get any better ideas that's what I'll have to do. All wires are just brand new and were replaced in post #31.

I did try a new coil. It didn't help any.

 

[cas3]

Has it been mentioned to run the car in the dark to see if you see any spark leakage from the wires or terminals?

 

[Jack E/NJ]

>>>[timing] light fluttered consistent with the missing sparks in cylinders 3, 6, 7, & 8. >>>
>>>timing light both fired consistently without any misses at all [from coil]>>>>

>>>Switching wires is, indeed, all there is left to do. Unfortunately, I don't have the crab cap that would make that so easy. >>>


Pull only one plug wire out of the cap. Start with #3. Then insert a short piece of known good plug wire into the cap that can reach a ground to make a spark. Start the car with 7 cylinders connected to the distributor. Does the spark from the short wire to ground seem steady or not? Use a timing light if you can't tell how steady it is. Do the same with 6, 7 & 8 one at a time.


This should rule out or rule in the plug wires. It should also tell you if the problem is further upstream particularly the cap & rotor.You've already tested that the coil output seems steady. So it's gotta be in the spark distribution system rather than spark creation system.

 

[Old Henry]

Has it been mentioned to run the car in the dark to see if you see any spark leakage from the wires or terminals?

Yes. I did that right from the start. (See post #1)
No obvious leaks from wires but a lot of the wires are hidden inside the conduits where there may have been leaks. Still, I eventually replaced them all.

 

[Old Henry]

Pull only one plug wire out of the cap. Start with #3. Then insert a short piece of known good plug wire into the cap that can reach a ground to make a spark. Start the car with 7 cylinders connected to the distributor. Does the spark from the short wire to ground seem steady or not? Use a timing light if you can't tell how steady it is. Do the same with 6, 7 & 8 one at a time.


This should rule out or rule in the plug wires. It should also tell you if the problem is further upstream particularly the cap & rotor.You've already tested that the coil output seems steady. So it's gotta be in the spark distribution system rather than spark creation system.[/QUOTE]

As I said in 3 posts ago (#70), I don't have a crab cap so can't swap wires around or pull one wire at a time from the cap to test it as would make sense. I will have to dismantle the entire wire loom to do anything like that. I may yet spring another $100 for another set of new wires like I just bought but wish there was a better way.

 

[1939mars]

You mention the generator and fan going on and off a few times in all of this and once that it ran fine before they went on. No idea how that could cause what you are seeing, but does taking that equipment off fix the problem?

 

[Jack E/NJ]

Sorry I don't mean swap wires around. Do you mean that you can't simply pull the #3 wire out of the cap's #3 receptacle?

 

[Old Henry]

You mention the generator and fan going on and off a few times in all of this and once that it ran fine before they went on. No idea how that could cause what you are seeing, but does taking that equipment off fix the problem?

I, likewise, had that same thought just today. That may well be my next move. Unfortunately, I can't drive very far without the water pumps and fan running but it's worth a try.

 

[Old Henry]

Sorry I don't mean swap wires around. Do you mean that you can't simply pull the #3 wire out of the cap's #3 receptacle?

Right. As you can see in the attached diagram, 42-44 had the crab distributor cap that would be easy to do what you suggest. But, the 45-48 has the sparkplug wires threaded through conduit, boots, and upper distributor cap, all before getting inserted into the lower distributor cap. Then that upper and lower cap are clamped together into one unit to attach to the distributor. No way to just pull one wire to do anything that I can see. You can't really do anything without removing the fan and generator and removing the entire wire loom.

 

[petehoovie]

Right. As you can see in the attached diagram, 42-44 had the crab distributor cap that would be easy to do what you suggest. But, the 45-48 has the sparkplug wires threaded through conduit, boots, and upper distributor cap, all before getting inserted into the lower distributor cap. Then that upper and lower cap are clamped together into one unit to attach to the distributor. No way to just pull one wire to do anything that I can see. You can't really do anything without removing the fan and generator and removing the entire wire loom.

 

[Jack E/NJ]

Aye!! I now feel your pain. If it was mine, I'd hafta run right out to the garage for my stash of used spark plug wires. Pull that cap cover and mess of wires crammed into those metal tubes. Then connect my old used wires directly to the cap and plugs to see if it runs OK.


BTW, I did away with those metal tubes on my helmet. Common sense suggests it's not good to tightly cram 4 or 5 high tension wires all together into a grounded metal tube. Replaced the tubes with black heater hose with a slightly bigger id for ease of assembly. If you don't look too close and maybe squint a little, you can hardly tell the difference.