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Sanity check on dual points conversion


z3beemer

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I've searched the forum on this topic but could not find exactly what I'm looking for. Here's the issue:  73 restoration.  Originally an auto trans with dual points.  I've converted to manual trans but I'm utilizing the original wiring harness.  All the pollution equipment has been removed.  The intake, carburetors, distributor, trans/clutch parts from a 72 have been installed.  Since the 72 dist. has only one points wire connection and the 73 had two, I've connected the two wires from the 73 wire harness together and connected them to the single connector on the 72 points.  Would this cause a problem?  The car starts ok and runs ok but is currently breaks up  after a few minutes driving and higher rpms.  Before I start more extensive troubleshooting to correct the issue, I want to eliminate the easy stuff first.  So, the questions are: connecting the two point wires from the dual system together and connecting them as one to the single contact point on the 72 distributor OK to do?  If not, which wire from the dual point harness should be connected to the distributor and which one should be disconnected? Your input is greatly appreciated.

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I’m not an expert nor have I attempted this before. My initial thought would be just to connect one wire and try that first. I’m sure you’ll find an answer here from someone that’s been down that road.

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I'm not going to verify my sanity nor anyone else's.

This may help explain the erratic behavior.

From the EE section of the FSM: (Note what it says about the dual points.)

image.png

So when the engine comes up to temperature, the timing is retarded for the automatic transmissions.

Here's how it looks in the wiring diagram:

image.png

As @Yarb said, use one set of points. Tune to that. Be happy.

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If I understand correctly, your modification should work fine as is and I'm thinking your issues are not (directly) caused by the single point mod.

So on the 73 harness, there are two black wires that go to the distributor for the points, right? And you have shorted both of them together and then connected that pair to the single wire coming out of the 72 distributor? If that's the case, it should be fine.

But, stranger things have happened. Can you put the original 73 distributor back in just to see if the problem goes away?

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Captain Obvious, your summary of my wiring is correct.  I was pretty sure my logic on how I connected it was correct, just wanted a second opinion.  Thanks.  I don't really think my current issue is ignition related.  I think it is more fuel related.  Maybe more carb tweaking.  I checked the plugs and 1-3 appear to be running rich, 3-6 appear to be more in the normal range.  Next I'll try leaning out the front carb and then rebalance.  Other possibility may be float adjustment but I think I was pretty accurate when I adjusted them.  I'll try the easy stuff first.

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Yeah, if it's as simple as it sounds, it's likely that your wiring change is not what's causing the problem.

However... Note the word "directly" that I used earlier.  LOL The reason I said that is because there may still be a problem with the 72 distributor you threw in there which is causing a problem. Points not set correctly. Dwell issue. Spark scatter from bushing wobble. Leaky condenser killing spark at higher RPM... Lots of problems could be caused by the distributor but might not be caused by the wiring modification you made. 

Point is, I'm not yet giving your 72 distributor a thumbs up, but I don't think the problem is your wiring change. Does that make sense?

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I guess a mechanical issue with the 72 distributor is a possibility but points, cap rotor and condenser are all new (not that that's a guarantee they are good).  Points were set using a dwell meter , not a feeler gauge.  Other things that have me leaning toward fuel issue is that it only happens under load.  At idle I can run the rpm's up to several thousand rpms with no issue. Also, if it was ignition related it would be odd that it only affected cylinders 1-3 and not 4-6.  Another fact that may be contributing to the problem (but not sure it would), currently, I do not have the entire exhaust system installed, only the headers are installed.  Planned on taking it down to the exhaust shop to get it finished out once I got it running decent. I still have the 73 distributor if I want to swap it out, I was unaware until this post that you could run a dual point distributor and use it as a single point set up.   Thanks for the input.  I appreciate it.

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It will pull high RPMs under no load. What happens when it's under load? Does it start to miss intermittently or just run out of power and refuse to rev higher?

 While not ruling out electrical, I'm leaning (snort) towards it being a lack of fuel.

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8 hours ago, z3beemer said:

I guess a mechanical issue with the 72 distributor is a possibility but points, cap rotor and condenser are all new (not that that's a guarantee they are good).  

Yeah, I'm not saying that's the problem, just tossing out ideas and possibilities. From the other stuff you mentioned, like differences between plug groups, it certainly sounds like it could be an issue elsewhere.

 Happy hunting, and hope you get it running the way you want it!  :beer:

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Mark it seem to be ok when driving it initially and at low rpm but started breaking up as I increased speed and rpms.  It's  ok at high rpms when idling with no load.  This was the first shakedown ride so I need to do some more investigating.  I think the next thing I'll try is leaning out the front carb a little.  If that doesn't help maybe recheck float adjustments again.

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The symptoms of breaking up worse under load do make me think of an ignition related issue. Your different colors on the plugs might be a tuning issue as well, but load dependent misfires would have me looking at ignition stuff first.

Plug wires arcing to ground somewhere, or coil wire arcing over to someplace it should not be going.

Have you put in new cap/rotor/wires? 

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We had a case here recently with high RPM misfires that turned out to be a poor fitting distributor cap that must have been moving around at higher vibration levels at high RPM’s. You could wiggle it with your hand, wouldn’t seat square and tight.  Replaced it with a new cap and viola as they say, revs to the moon with no mis-fire.   Ignition systems rely on a in-spec gaps and fitment, any “Extra” energy lost with a too large gap (or inconsistent gap) and resulting spark, results in a weak plug spark and mis fire. 

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2 hours ago, zKars said:

We had a case here recently with high RPM misfires that turned out to be a poor fitting distributor cap that must have been moving around at higher vibration levels at high RPM’s. You could wiggle it with your hand, wouldn’t seat square and tight.  Replaced it with a new cap and viola as they say, revs to the moon with no mis-fire.   Ignition systems rely on a in-spec gaps and fitment, any “Extra” energy lost with a too large gap (or inconsistent gap) and resulting spark, results in a weak plug spark and mis fire. 

This goes back to a thread somewhere on here about using an oscilloscope to diagnose ignition issues. I need to play around with that some more. You need an attenuator on your voltage input. That gets connected to the negative terminal of the coil.  It helps to use an inductive probe to put on the plug wire for #1. That will serve as the trigger for the scope, and you can trace the signal from the coil voltage to a cylinder.

Larger spikes on the coil voltage trace when the ignition is firing is indicative of having to overcome a larger gap. That gap could be at the rotor button, coil HV wire, or plug wire. There are videos on YouTube that explain this better than my quick summary.

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 The only time (in twenty plus years of spirited driving) I ever experienced an erratic misfire at high RPMs was cured with a new set of BP6ESs. My experience with a partially plugged fuel filter resulted in the car refusing to go above 45 mph in any gear but would freely rev to 6000 rpms under no load.

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I thought of another mis-fire problem and the solution. Like Mark, it was related to the plugs. 

I used, and still use, NGK BPR6ES plugs. The issue I routinely run into with them, is that the end of the plug where wire snaps on, is threaded on to the plug, and is often loose. 

Several years ago (2015), a random issue with mis-fire throughout the range dogged me for a year or better, and when I finally discovered that one of the end caps was loose on a couple of plugs. Tightened them up, and the mis-fire went away. 

The BPR6ES-11 plugs that also spec'ed for 280's with EFI, don't have the threaded end cap. 

I described this WAY back in 2015. The thread lionk is below if you want to see pics.

 

EDIT EDIT : just doing a bit of a good search. NGK plugs BPR6ES  #7131  have the threaded caps, #4008 have solid caps! Jim, stop buying #7131 BPR6ES plugs you old idiot!

 

BTW, Don’t order the Denso #4008 plugs by mistake. They are NOT the same….. Just saying…..

Edited by zKars
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Well, it appears that the problem was the coil.  I replaced it, and took several rides and problem appears to be gone.  I also took your advice and ordered new NGK one piece plugs, but haven't got them yet.  The finish line is in sight.  Thanks guys.

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Glad you got it running well.

However, just to make my little brain good with the whole ordeal... That same coil ran just fine with the original dual point distributor? Some ideas?

Been a long time since it ran at all and anything could have happened between then and now.
The coil was borderline before and your moving stuff around and angered the electrons in the coil.
Your car runs fine now and it just doesn't matter why changing the coil fixed it.

LOL

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Never really ran it much with the dual points.  It did start but it was intended to be a restoration project since I purchased it.  All I can say is that it ran, not well but It had so many other issues I didn't worry about it too much. Thanks again to all for your help.  I have a few more minor gremlins to deal with but close to the finish line.

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