MSD rough run on 82 911
 

i've run into this before and just pulled it off and gone back to the stock ignition system and it was like instant tuneup. the car starts to pop and fart around 3K. the MSD tech line guy said it's common with pointless distributors like on an SC. but i'm sure there are many running around that work fine, or were made to function properly with the addition of some other electronic component, like a resistor or something. is there anybody out there who has experienced this problem? it's a 6A with a bosch blue coil, and jacobs carbon core wires. customer installed, but everything looks correct.

Sounds like the MSD switchover circuitry to go from multiple sparks to single spark has flaked-out.

One of our club memebers cars was acting just like you described. He took his brand new Jacobs wires off and put the originals back on and the prblem was solved. Just my $.02

the switchover circuit sounds a likely candidate. maybe it's just a defective unit. the car ran the same before the jacobs wires, according to the owner. it ran well until the bosch unit went bad and was replaced with the MSD. it'll be out of there today, unless i get a brainwave.

What benefit is the MSD suppose to have over stock on a 911SC? I put 6A on a 66 Corvette and it made a huge difference, but at the same time I took out the points and replaced with Pertronix system. Seems like there would not be any benefit with MSD over stock ignition.

i never felt that there was any improvement over stock. that bosch CD kicks ass.

Noah, Do you feel any performance gains in the higher RPM's? Where if felt a difference on a vette is it ran much stronger at redline. Knock on wood, my 911SC starts and idles pretty good.

Some trusted folks have told me that MSD is an improvement over stock, but I am still skeptical. I am skeptical because I cannot understand how multiple sparks can help at all. Either the fuel ignites or it doesn't.

It costs less if your factory CDI dies.

Tom

It definitely works better on an early car with Webers. Starts twice as easy and idles better. I have done head to head tests - switching each out just to convince myself.

I'll second the benefit to Weber equipped cars. Really smoothed out the idle.

Also the MSD combined with their coil lets you run a bigger plug gap. My 73 is running .045 and it runs much better than it does with the stock gap.

Here is the part that is tough to swallow - I think the MSD helps by compensating for other problems. Poor mixture? Marginal wires? Marginal compression? Vacuum leak? No problem, slap on an MSD and one of the multiple sparks is sure to fire the mixture and smooth it out. Never mind that the root cause of the problem remains.

In other words, MSD probably doesn't do much for cars that are running perfectly. It merely band-aids cars the have problems. And it turns out that old worn out webers are hard to get tuned perfectly, so MSD helps them (hides them) a bunch!

The MSD on my turbo motor made it run better during warmup, but that's it. No difference once warm or at full throttle.

Is he using the violet and purple wire to trigger the MSD?

Reversed polarity will give alot of advance as per MSD instructions.

When i installed on my 3.0L conversion, the polarity was reversed but i didn't figure it out at first. I had wired it per instructions figuring that the center of the green wire from the dizzy was + and that the sheilded part f the wire was negative.

I didn't have a Bosch CDI so i could'nt compare. I tought at first that the dizzy was off a bit (i had taken it out) so i reindexed the dizzy to be able to get the advance in spec.

Car still wasn't running ok.

At one point I looked at another 3.0l. We put it to TDC for #1. I then noticed that the reluctor (star thingy) on his was real close to the external posts compared to mine.

I reindexed my dizzy again, this gave my alot of advance again. Switched the purple and green wire and it ran much better.

I'm using an MSD6A box (not the AL with the softrev), using an MSD high vibration coil and a set of magnecor wires.

HTH

Marc-Andre
70 914-6 conversion 3.0L Webers

Sounds like some symptoms I had before with a 6a I had on a SAAB Turbo. Turns out the unit was going bad. Spark was progressively getting weaker and weaker, giving the car problems at higher revs and loads, until I pulled out the unit and reverted back to the stock Bosch CDI unit. Problem solved, and the MSD unit still sits in my garage today.

well, tried the polarity reversal yesterday, and no diff. so the bad tooth has been yanked this morning, and a permatune installed. runs normal now. but keep the ideas coming, because it won't be the last MSD that runs wierd.

I agree with Chuck about the msd hiding a lot of carb problems. It certainly has improved the idle and low rpm running of my motor. If everything was dead on I would think that the MSD wouldn't make any difference. Every thing is dead on in perfectland, this is MSD Land. (Paraphrasing Walgreen commercials)

I believe that the multiple circuit switch over is starting to go bad on the msd unit that you have John.

Good luck,
David Duffield

BTW - MSD's improve the low end running on early MFI cars too. It's not just a carb thing.

Also; MSD's do go bad. They don't have perfect reliability. Did you ever notice that good TransAm teams and NASCAR teams running MSD's (which many do) will often have two of them side by side with a 2 way switch so that if one fails they can immediately switch over to the second one.

JW,
I had the same problem when I installed my
MSD unit. I took the car to Bob Norwood, he
put a MSD coil on it. Has run like a bat out of
hell ever since.

Warren had it right from the get go, I mentioned this in another thread awhile back since I've have a lot of experience with MSD's equipment. If the problem is at 3000 then it's the crossover from multiple to single spark 9 times out of 10. I can also second from experience the benefits for carbs and the use of an MSD coil along with wider plug gaps, mine are .055" I used to think the trouble with MSD's failing was due to vibration but I have their 7AL3 box in my drag car and the idle vibration from solid mounts and a HUGE cam rattles so bad it required weekly dental work LOL :D My only other thought is that MSD's have only failed on me when mounted in the engine bay, they have never failed when mounted in the passenger compartment, something to consider FWIW. ;)

before my P days I was setting up a '71 Camaro to be worthy of a great engine.. so in the prep stages I wired in a MSD on the old/beat/high miles engine.. it idled smoother, started better, ran cleaner. that car is still around 8 years latter.. it's always keep outdoors within 1,500 ft of the Atlantic Ocean. MSD is 10+ years old .............Ron