Mystery PerTronix ignition issue

Unknowing

New Member
Sep 29, 2020
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NewJersey
Hey y’all, new guy here. '66 Mustang—289: I Had a mechanic install a new PerTronix distributor and coil, wires and plugs. Since I’ve gotten it back, there is a periodic momentary stumble, but the rest of the time it runs fine. It usually happens when it's warmed up, in gear, at idle, such as at a stop light. It will act like it’s going to die, then recover and just continue running as normal. A couple of times it has actually died, but then starts right back up. I’ve checked the fuel filter and carb filter—- seems to be getting fuel okay. I’ve checked all the wires, including the wire to the ignition switch and the battery cables, and everything is tight. Any advice? What am I missing? I appreciate your reading this.

PerTronix Flamethrower III distributor and coil
NGK Iridium plugs
 

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MustangIIMatt

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Mar 7, 2002
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Did the mechanic set the timing and vacuum advance?

Also, you don't need iridium plugs in a 289, next time you need them, just get the NGK V-Power plugs, they'll be fine (they're actually what I have in my Mustang II's 308 right now).
 

Unknowing

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Sep 29, 2020
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NewJersey
Yes sir. Timing is a mystery to me, so I’ll tell you what he told me when I took it back with this problem. He said that hed set a three degree advance, but maybe that was too aggressive and was causing the stumble, so he said he would retard it three degrees and to see how that worked. Thank you for the advice on the plugs. Will they be okay, or should I swap them out? Sounds like what you’re saying is that I bought too robust a plug for this particular application. Live and learn. Thanks again.
 

MustangIIMatt

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Mar 7, 2002
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Yes sir. Timing is a mystery to me, so I’ll tell you what he told me when I took it back with this problem. He said that hed set a three degree advance, but maybe that was too aggressive and was causing the stumble, so he said he would retard it three degrees and to see how that worked. Thank you for the advice on the plugs. Will they be okay, or should I swap them out? Sounds like what you’re saying is that I bought too robust a plug for this particular application. Live and learn. Thanks again.
They should be okay, just replace them when they need it. Iridium plugs are designed for modern engines with EFI. They've typically got much smaller electrodes than copper plugs, and are designed to last 50,000-100,000 miles in modern vehicles (some manufacturers including Hyundai and Toyota don't even have a recommended change interval for them in their new cars). A carbureted engine isn't going to run cleanly enough to get the full benefit of platinum or iridium tipped spark plugs, so there's just not a point in wasting the money on them, nickel or copper plugs are just fine for your application. I can't recommend NGK enough for spark plugs in general though, their spark plugs are absolutely top-notch, and I install them in virtually everything I do spark plugs in, both at home and professionally.
 

zookeeper

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Aug 25, 2001
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I'm not sure what you're mechanic was talking about with the timing, but here's the method I use: start the car. If it starts, you're good to move to step 2, using a timing light (preferably with a tach) bring the engine speed to about 2700 at which time you'd like to see about 32-36 degrees total advance. Let the engine return to idle, and shut it off. Then disconnect the vacuum line from the advance canister and plug it off at the carb to avoid a vacuum leak. Recheck your timing at idle and you should see maybe 6-10 degrees BTDC. Reconnect the vacuum line and drive it, it should be about right. If it pings under load, it's too far advanced, if it overheats, it's too far retarded. Timing has a huge affect on power and driveablity to take the time to get it right
 
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wicked93gs

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Sep 30, 2006
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Nashville TN
They should be okay, just replace them when they need it. Iridium plugs are designed for modern engines with EFI. They've typically got much smaller electrodes than copper plugs, and are designed to last 50,000-100,000 miles in modern vehicles (some manufacturers including Hyundai and Toyota don't even have a recommended change interval for them in their new cars). A carbureted engine isn't going to run cleanly enough to get the full benefit of platinum or iridium tipped spark plugs, so there's just not a point in wasting the money on them, nickel or copper plugs are just fine for your application. I can't recommend NGK enough for spark plugs in general though, their spark plugs are absolutely top-notch, and I install them in virtually everything I do spark plugs in, both at home and professionally.
Honestly, there is no time that platinum or iridium plugs haven't given me a problem, modern car or not, I have traced so many misfires to platinum plugs its not even funny...I don't bother to wait for them to start misfiring anymore...first thing I do with a new car is change to good old copper plugs....in general you only replace them a few times over the time you own a car anyway. Other people swear by platinum plugs....but all they ever do for me is cause misfires.
 
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MustangIIMatt

I need something stupid to play with
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Mar 7, 2002
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Honestly, there is no time that platinum or iridium plugs haven't given me a problem, modern car or not, I have traced so many misfires to platinum plugs its not even funny...I don't bother to wait for them to start misfiring anymore...first thing I do with a new car is change to good old copper plugs....in general you only replace them a few times over the time you own a car anyway. Other people swear by platinum plugs....but all they ever do for me is cause misfires.
Autolite, Bosch, E3, or Splitfire?

Those are the only plugs I've heard of causing misfires in modern engines even when the correct material was used.

Some of your modern engines (especially in high-end machinery from BMW, Mercedes, etc.) will actually throw a misfire code if a copper plug is used. The DME (German acronym for PCM) is looking for that specific of a combustion event that even though there's no actual misfiring, it'll throw a misfire code.
 

wicked93gs

10 Year Member
Sep 30, 2006
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Nashville TN
Autolite, Bosch, E3, or Splitfire?

Those are the only plugs I've heard of causing misfires in modern engines even when the correct material was used.

Some of your modern engines (especially in high-end machinery from BMW, Mercedes, etc.) will actually throw a misfire code if a copper plug is used. The DME (German acronym for PCM) is looking for that specific of a combustion event that even though there's no actual misfiring, it'll throw a misfire code.
Nope, just standard NGK platinum plugs. Its just my experience in the end. There is nothing wrong with copper as long as the electrode isn't worn away. Another thing some plugs have that will cause misfires are those stupid little screrw-on caps on the end...they can vibrate loose and start misfiring as well.
 

MustangIIMatt

I need something stupid to play with
Mod Dude
Mar 7, 2002
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Nope, just standard NGK platinum plugs. Its just my experience in the end. There is nothing wrong with copper as long as the electrode isn't worn away. Another thing some plugs have that will cause misfires are those stupid little screrw-on caps on the end...they can vibrate loose and start misfiring as well.
I hate those little screw-on caps. I get it, they let the plug fit other applications, but they suck.
 

Unknowing

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Sep 29, 2020
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NewJersey
Thank you guys for your information. I'll set the timing from your guidelines Zookeeper. I haven't had any misfires from the NGK plugs yet, but I'll keep the info in mind.

In regard to the original problems with the stumble, I found out that the new Petronix Flamethrower coil has to have a full twelve volts from the ignition. The original ignition wire is a resistor wire that only provides something like 9 volts. I stuck a relay on there that I ordered from Petronix and it runs like a champ.