Spark plug wires sparking in the dark - source of misfire
 

Looking for some input here. I have been chasing down a persistent misfire on my 78 SC (80 SC engine).

The setup: 1980 SC engine with a top end rebuild 20K ago. Converted to Megasquirt 2 - fuel ratios appear stable. Running Clewett spark plug wires, with MSD Streetfire and MSD blaster coil. Plugs are NGK BRP7EIX with standard gap. Using a standard ignition rotor (with a resistor) in the rotor.

The Situation: Engine generally runs well but at 2800 to 3200 stay state a misfire/stumble sets it that I can't seem to eliminate. There is also an odd vibration when coming off the throttle at similar engine speeds. I've checked ratios, MSD wiring, plug connections, grounds, sensors, etc. with no change in outcomes.

Tonight I turned off all the outdoor lights and looked in the running engine compartment and there were a myriad of tiny sparks coming off plug wires primarily on the drive side of the engine. The Clewett wires are only 4 years old - could they be bad? Passenger wires spark slightly nearer the coil but not near the right side or the plugs. The sparks are not huge but there must be at least 15 small points of sparking.

1) Is this the likely cause of my misfiring? Why would 4 year old Clewett wires be doing this? Does the fact that I didn't increase the gaps on the plugs force the sparking?
2) I thought larger plug gaps would increase the resistance and would likely force the sparking?
3) Does using a resistor rotor increase the likelihood of sparking/misfire?

I don't want to just throw money at parts but would replacing the plug wires be a safe bet, and what would you recommend for wires?

Looking forward to your input folks.

Mark

I would put the plugs back to spec and see if the wires still spark or check the OHM on the wire set.

Next would be is to go back to OEM Berus, I did from Magnacors back to Berus due to wires not seating properly on the plugs.

When you say put the plugs back to spec, do you mean use stock plugs or the factory plug gap? I’m currently using the factory gap on the NGK plugs I have installed.
Interesting thought on the Beru wires.

I thought I read you did not increase the gap, so you are at factory gap? Good.

Then I would remove the wires and check the OHMS on the wire set. Obviously, there are issues with those wires. Are you sure you are fully seated on the plugs and dizzy? On the plugs you should hear/feel the click and on the dizzy, insure there is no air trapped under the boots. Use a pick tool to probe under the boot and try the night test again.

The problem with my Magnacors was it kept popping off #2 plug. Got sick of pulling over to re-seat the plug. Back to Beru, no more issues.

Part of my standard install setup for plug wires is to seal the whole system with dielectric grease. In addition to the normal sealing of the boots to the plug and coil, it also includes sealing the boots to the wire. Push the boot just a touch up the wire and grease the seam and then push the boot back down.
A well sealed boot should hold air pressure which will cause the boot to pop off. You need to pinch/squeeze one side of the boot to burp the air out of it so it will say seated. The plugs are a bit harder. Rocking it from side to side helps some but what really solves it is going through a heat cycle to bleed the trapped air out from under the boot.
A quality set of wires prepared like this will run under water. I have a set of 15 year old Magnecor wires that have never had an issue through rain, car washes, and any other abuse.

But...
The second you start to get sparks jumping you now have a problem with carbon tracks. Once you have a carbon track started it is likely you won't be able clean it off and if you can't remove the alternative path you have to replace the part.

If your wires are good, you should be able to give them a light spray of water without any sparking or stalling. If you can see spark leakage dry, those wire are toast. There is a local guy that was making up wire sets using your ends, PM me if you want his contact info. Also, it's not too hard to do it yourself, I do have some extra wire "in stock".

Mark,

I had a similar problem a few years ago. The OEM wires had failed, I went with Clewitt wires and it ran great…until it rained. Replaced those with Magnacor. Problem solved. I was too cheap to buy the OEM ones in the beginning and ended up spending a bit more overall.

If you’re seeing sparks, the wires are bad.

Mike

Do you have wire separators installed? I just installed a set of Clewitt wires on my 86 and found that the factory wire holders and separators didn't fit the fatter Clewitt wires. So, I went to a FLAPS and bought some generic holders from the hot rod section of the store to keep the wires properly separated. If the wires aren't separated misfiring can occur due to inductance.

I have never seen the little electric light show unless the wires are bad.

I agree with the previous responses that when you start seeing arcing the wires are bad. In regard to checking the resistance of the wires; at this point it is irrelevant. the core conductor could be well within speck but in your case the failure is of the insulation not the conductor.

Here you go, how to check resistance for the wire set:

http://forums.pelicanparts.com/porsche-911-technical-forum/1007675-multimeter-help-needed-spark-plug-wire-resistance.html

Part of the problem is the stupidly high voltage (~500V) that MSD use on the coil primary. This results in 50,000V on the plug wires. The higher the voltage, the more likely it is to ‘jump out’ of the wires (or take an easier path to ground). If the wires are close together (like over the fan housing) this can even result in firing the wrong plug!

Porsche CDI systems use a mean voltage of around 30,000V and the components are designed around this. Sometimes more isn't necessarily better.

Thank your for everyone's replies. Based on the responses it seems that:
The wires are toast if dry sparking.
Wires should be well separated and sealed - which is currently the case though minor sparking was indeed seen at a point on top of the shroud.
The Streetfire might operate at an unnecessarily high voltage, perhaps provoking sparking.

So:
New wires Magnacor or locally sourced - Brighton911 please pm me the contact info.
Reinstall the original CDI - I would like to go Classic Retrofit but will need to rebuild the war chest Jonny.
Seal and separate the wires well.

Mark

If you have your old Beru wire set, take the dust boots off and use the Magnacor don't last. Ask me how I know. Maybe they changed but, it is what I experienced with the dust guards/boots

Another thing is Magnacors are thicker so be aware the wire holders may not fit right.

Sowwy to be a downer :-( on the Magnacors. I really loved them in the beginning.

Use the pick tool to get all air out of the plug connectors on your dizzy and use some dielectric grease to make sure they are properly seated.

Break a Leg Brother!

Installed the Magnacors and they are staying on the plugs :D. Significant difference in idle, at speed and off throttle. My misfire seems to have been cured and as an added benefit pops and bangs on decal and part throttle have backed off to an appropriate burble. I should have cleaned the plugs when I installed the wires just to eliminate any fouling issues but I was too eager to see if new wires would have an impact. I think I'll now be able to focus on the fine tuning of the MS 2 given the reduction in other running "noise". Thanks everyone for their input.

Mark

Jonny, You beat me to it. Plug gap does not adequately control the voltage with a CDI. The voltage needs to be limited and controlled at the source, as your CDI does. Fred

The takeaway on the CDI discussion is either a well serviced and functioning Bosch CDI or Jonny’s CDI+ are more than up to the task. If I chose to continue with my MSD for the moment I am probably best keeping the stock gap on the plugs so that voltage Is dispersed more efficiently than with a larger gap. Jonny and Fred you might have to edumacate on this latter point.

Mark

Marwil - you probably know that DIY autotume recommends Magnacore for the MS. I just can't get an answer to which product.

What did you use? 7mm, 8mm, KV85? or other?

thanks,

Targa44 -Nope, didn't know that, looks like I stumbled in the right direction. I used the KV85 wires and didn't have any trouble fitting them.

I had a similar issue a few years ago, with a similar conclusion. Long story short, buy the more expensive OEM wires/gears/rings/whatever or you'll spend the difference and more in time and frustration.