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71_dart_swinger

71_dart_swinger

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So. Car is 440 with msd 6al2 digital. The other day. Changed my plug wires I have the Taylor 409s. After car starts up. Idles normal. After a slight warm up. All of a sudden the rpm will jump up to about 1300-1400. And the tach will jump / rev in reverse and the engine will pop and back fire. I turn car off and immediately restart. It runs fine for 5-10 minutes. Then same thing. Just put a new msd blaster ss coil because I thought it was that. ? Any ideas. Thank you
 
71_dart_swinger said:
So. Car is 440 with msd 6al2 digital. The other day. Changed my plug wires I have the Taylor 409s. After car starts up. Idles normal. After a slight warm up. All of a sudden the rpm will jump up to about 1300-1400. And the tach will jump / rev in reverse and the engine will pop and back fire. I turn car off and immediately restart. It runs fine for 5-10 minutes. Then same thing. Just put a new msd blaster ss coil because I thought it was that. ? Any ideas. Thank you
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I disconnected the tach signal wire.. thinking burnt tach or something faulty. Bid same thing. Car has 13 something almost 14 v at idle
 
Loose cap? Sticky mechanical advance? Cracked vacuum hose?
 
Two thoughts...
Do you have the old plug wires? Try putting them back on and see what happens

Are you 100% sure you got the firing order correct?
 
If you google this it is likely EMI AKA RFI (radio frequency interference.) The high output power going through the plug wires can actually trigger the MSD. Do a search and make sure your new wires are compatible with MSD. Then, check the routing of the plug wires close to other, low voltage and trigger wires, like, the trigger from your dist feeding the MSD. Make sure the trigger and other sensitive wiring is run close to the firewall or other body metal "ground plane", keep the plug and coil wires away from them, and cross them at near right angles. Do not run plug wiring parallel to sensitive trigger/ sensor wiring.

This type of deal has developed into a whole science due to EFI, sensors, triggering circuits, and other very low power circuits next to high power magnetic fields
 
67Dart273 said:
If you google this it is likely EMI AKA RFI (radio frequency interference.) The high output power going through the plug wires can actually trigger the MSD. Do a search and make sure your new wires are compatible with MSD. Then, check the routing of the plug wires close to other, low voltage and trigger wires, like, the trigger from your dist feeding the MSD. Make sure the trigger and other sensitive wiring is run close to the firewall or other body metal "ground plane", keep the plug and coil wires away from them, and cross them at near right angles. Do not run plug wiring parallel to sensitive trigger/ sensor wiring.

This type of deal has developed into a whole science due to EFI, sensors, triggering circuits, and other very low power circuits next to high power magnetic fields
Click to expand...
It’s just so random. But ya tomorrow I’ll throw the old wires on just to see. Wires are compatible. Ran the last set for about 3 years. No issues. Will move the wires from cylinder 6,8 as I ran them around back if valve cover and next to the trigger for the dist. So that may make sense. Thank you. Will definitely look into it. Would it take a certain amount of time before it would affect the trigger. As it runs fine for 5-10 mins before it changes ?
 
I would move the wires ( as 67dart273 pointed out) before swapping any parts, that way you will know that that was or was not the issue
71_dart_swinger said:
Would it take a certain amount of time before it would affect the trigger
Click to expand...
Might be that the various wire insulations break down with temperature, just enough to cause issues?
Or maybe the electronics become more sensitive once warmed up?
 
I'm a little confused;
if you shut it off while the idle is high, and then immediately restart it; what happens?
According to post #1

71_dart_swinger said:
It runs fine for 5-10 minutes. Then same thing
Click to expand...

Is this with a carb?
Do you have variable duration lifters?

I don't see the tach running in reverse, I see it dropping to zero immediately after the blip, as if the ignition was shut off, and immediately it recovers. And the engine seems to be doing the same thing; so my guess is that the tach is actually telling the truth?
The AF gauge seems to be echoing as the numbers go very very lean during the stumble.
Ima thinking you have multiple issues;
1) does the cold engine idle rich,then over time gets air somewhere other than the primaries, which increases the idle speed
2) the engine cuts out on tip-in, and
3) I can't figure out the electrical cutout
 
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AJ/FormS said:
I'm a little confused;
if you shut it off while the idle is high, and then immediately restart it; what happens?
According to post #1



Is this with a carb?
Do you have variable duration lifters?

I don't see the tach running in reverse, I see it dropping to zero immediately after the blip, as if the ignition was shut off, and immediately it recovers. And the engine seems to be doing the same thing; so my guess is that the tach is actually telling the truth?
The AF gauge seems to be echoing as the numbers go very very lean during the stumble.
Ima thinking you have multiple issues;
1) does the cold engine idle rich,then over time gets air somewhere other than the primaries, which increases the idle speed
2) the engine cuts out on tip-in, and
3) I can't figure out the electrical cutout
Click to expand...

correct. I start it. It runs fine. After a few minutes it’ll act like a switch was turned on and the idle will jump up. And yes. If I turn off ignition and immediately restart. It will do same thing. Run fine for a few minutes. Then start missing again .
The car runs a tad rich at idle. But not enough to foul anything. I dont believe it’s getting air from somewhere else?
 
Could you have crossed some new wires?? Or I'd try putting the old plugs and wires back on "IF" that's all you changed. If the problem goes away then you know what it is.
 
The tach acting as it is in the second video HAS to be an electrical problem. No carb/ fuel problem could cause a tach to react that fast and in that way. I still say it is most likely electrical interference. It is ALSO possible.........if EMI is the problem........that you damaged some electronics in the system. I would not necessarily trust AF readings either, as that system can also be affected by EMI
 
That pop back through the carb could have taken out the power valve diaphragm if you're running a Holley. Would make it rich everywhere.
 
what if you shielded the tach cable with a ground sheath, ie. BX or MC cable. 14/2 would be workable in an automobile app. Its a helical armor wrapped 2 conductor cable. Would not look bad either as its shiny and would provide a nice RFI shield when you ground one end.
ACTHH-image-for-spec-page.jpg
 
There's lots of good advice here.
Why did you change the wires in the first place?
Unless they were old and dirty or had continuity issues or if the car didn't run good with them before, then why bother with the expense and hassle of changing them?
I would put the car back exactly the way it was before and see if it has changed since then, and if it has, then you need to focus on the new problem.
This might just be a case of what happens if you fix something that ain't broke..........
(Nice sounding car BTW)
 
pishta said:
what if you shielded the tach cable with a ground sheath, ie. BX or MC cable. 14/2 would be workable in an automobile app. Its a helical armor wrapped 2 conductor cable. Would not look bad either as its shiny and would provide a nice RFI shield when you ground one end.
View attachment 1715555978
Click to expand...

I've brought this up before. shielding for this kind of thing is normally known as a "Faraday shield." You ground it on one end only. One way to do this (in the telco/ data world) is let's say you have some CAT5 and are only using 1 or 2 pair. You tie all the unused pairs together at one end, in this case the MSD box end...away from the distributor and ground ONLY that end. You could scare up some 2 wire shielded cable (They make it, both specialized RF coax and for use in the audio industry, "balanced" microphone cable) Same deal, ground only one end of the shield
 
I snagged some 24/2 shielded cable from my telco central office for my crank sensor run, not sure what we use it for...it was orange.
 
Could use new dist cap. But I know that can’t be this problem the cap alone. There is a popping noise coming from the cap while running. Part of the inside of distributor is rusted? I’ll post a pic if I can. The wires are not crossed anywhere. Not even close to anything . The old wires were melting and brittle at the plug boot. Super tight fit with my headers I have heat boots on the wires. But they were 4 ish years old. There’s definitely a popping coming Fromm inside distributor. I’m just wondering. What is warming up to a point we’re it causes the problem. If it was electrical interference? If think it would interfere from start up. Not 10 minutes later. But idk. Lol. I’m thinking if I popped Power valve. The next time I’d start it. It would run rich from start up. But that’s not the case ? Tinkered with it a tad today. Idled for 10 minutes. Did small week burnout Felt under powered. Drove it for another 3 minutes. Then it just started up again. Turned it off. Started. Drove it a mile. Same thing. Then lumped it home. Lol
 
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