1983 SC w/MSD 6AL surging over 3000 rpm

[walt83sc]

1983 Porsche 911SC 3.0L CIS, mostly stock.

Over a year ago, car started having issues; it ran fine, but would die when hot. Shut it off 15 minutes, and it would start up and run fine, then drive another 20 minutes, it would die again. This happened 3 or 4 times before it finally died completely. It wasn't getting a spark. The CDI was a Permatune, so it had been replaced by a PO.

So, it sat over the winter, then I put in a 6AL, Blaster coil, 8920 tach adapter, and also replaced plug wires with a new set of Clewett wires. Car wouldn't start... Good spark, so I suspected no gas. For grins, I tried arming, triggering, and then disarming the factory alarm (factory alarm disables the fuel pump when triggered, from what I've read).

Then, the car decided to start, and idles fine now. (In fact, it idles better than it ever has, and the cold-start problem it used to have is gone.) Adjusted timing to 5 deg btc, per factory specs. Then I rev'ed it up...

Above 3000 RPMs, it surges rapidly to 5000 or 5500, then cuts out, drops to <3000, then surges again. It seems fine up to a little past 3000 RPM. I recall that at 3000 RPM the MSD drops back to single-discharge (vs multiple discharge). Coincidence? It will actually hold steady up to about 3300 RPM, if I can trust the tach - which seems to be working fine.

I'm wondering if it might be the fuel filter after sitting for around 9 months... I've got one on order, since I have never replaced it, seems like it's probably overdue. It has 3/4 tank of last years gas, too. Since it seems to run fine up to 3000, I don't think it's gone bad. I've run it for probably 15 or 20 minutes, mostly idling.

Any other suggestions would be greatly appreciated. I also removed the A/C compressor, since it wasn't working, and replaced the oil pressure sending unit while I was in there, but I don't think either of those are related.

Thanks
Walt

[VaSteve]

Was it really *surging* or was it just the tach jumping. Or was it a light surging though the tach would indicate more? When you would come off a cruise around 3000 rpm to idle, would the tach hang?

Pop off your distributor cap and take a look at the rotor. See if it's melted.

[walt83sc]

Thanks for the reply. I haven't driven it yet. I was just manually opening the throttle. The engine was definitely surging. I'll have a look at the rotor but I think the lower rpms would be affected if that were the case. Thanks again

[James Brown]

Small point, you should not need the tach. adapter with a 83 electronic dissy, but it won't hurt. Put a timing light on one of the plug wires and do the same, rev it up and if it dies check if you still have spark. Also around 3K, the timing advances, check to see if the mechanical advance is working and/or if the vacuum advance (and retard-but not needed) is working. If the electrical is OK and there is a rev limit chip in the 6AL (6-7k), then start looking at the fuel side.

[walt83sc]

So, the rotor is fine, and the MSD has a 6000 chip in it. The timing is definitely advancing, and the spark maintains during surging (doesn't cut out when RPMs fall). Guess I'm into the fuel filter now. From what I've read about the accumulator, it doesn't seem likely that is involved. As an aside, I replaced the fuel pump last year, so it *should* be ok. Thanks for all the great suggestions so far.

[JP911]

You don't mention how you wired up the MSD, but if you get the purple and green wires wrong (go to the distributor green wire) you can get a rough running situation similar to what you describe.

[walt83sc]

Replaced the fuel filter, with no change. Under load (while driving) it revs to 3400 RPM then cuts out.

[Bob Kontak]

Refer to JP911's comment. You may have wired something wrong. Also the ground wires for the MSD box need to attached in a way that people would accuse you of being a complete anal retentive fussy-butt. I attached my ground to the body lug for the transmission ground strap.

The ground advice is from Steve Weiner at Rennsport Systems.

[VaSteve]

Originally Posted by JP911

You don't mention how you wired up the MSD, but if you get the purple and green wires wrong (go to the distributor green wire) you can get a rough running situation similar to what you describe.

Also, I seem to recall that the MSD requires you to cut one of the wires if it's not a V8. Did you cut the wire?

[walt83sc]

Ah, good point. MSD wiring:

heavy red wire - connected to hot wire on starter (this took the longest)
heavy black wire - connected to aluminum plate that MSD is mounted to
thin red wire - connected to pin 15 (red wire) in CDI plug
thin black wire - connected to negative side of coil
orange wire - connected to positive side of coil
purple wire - connected to 31d (thick black wire) in CDI plug
green wire - connected to 7 (green wire) in CDI plug

8920 wiring:
red wire - connected to pin 15 (red wire) in CDI plug
black wire - connected to aluminum plate
white wire - connected to what I presume to be tach output of MSD box (recessed male spade)
purple wire - connected to TD (thin black wire) in CDI plug

I did cut the red wire on the MSD.
Plugs are at stock gap (.028)

I'll double check the wiring tomorrow and measure for voltage drop between the aluminum plate and the battery.

Thanks everyone.

[walt83sc]

It does seem to run fine up to 3400 RPM... no missing or hesitation. I checked the rotor also. It was a bit hot (150 deg F) according to my laser-guided infrared thermometer, but it wasn't melted or discolored. I had the dizzy rebuilt a couple years ago due to excessive play (rotor was whacking the contacts inside the cap), so the cap and rotor are in very good condition.

I also need to check that large, black connector at the rear of the relay panel; I've had trouble with that in the past, and it was certainly disconnected at least once while installing the MSD. One of the connections over-heated and lost contact a year or two ago. I had to resolder the male pin back onto the wire, but now thinking I need to re-wire that circuit to bypass the plug. Now that I think about it some more, it seems like the power for the rear blower runs through there, and the rear blower overloaded when a screw loosened and hung up the motor, so maybe it's not all that critical. Worth checking, nonetheless.

[Bob Kontak]

Suggest the black wire should go to a better ground. Here are the instructions for the black wire from the MSD install document:

This wire connects to a good ground, either at the battery negative (-) terminal
or to the engine.

I would normally think the aluminum plate would suffice but running a jumper to an engine bolt may be worth a try.

[JP911]

Sounds like you have the purple and green correct. Two more things to look at, in addition to the grounding improvements: 1. is the green wire at the distributor in good shape, 2. re-gap the spark plugs per the MSD specs (I can't remember the 911 recommendation, but if you do a search here you'll find it easily).

[vash]

i think mine are gapped at 0.040..maybe 0.045. i seem to remember a thread where someone couldnt get the car to run correctly and a wider gap fixed it...( or i may be needing a second cup of joe)

[Bob Kontak]

JP911 - I will push back on the suggestion to take the plugs to the full MSD gap based on my experience.

The larger gap will exploit any connection weaknesses in the secondary system. Essentially, the spark will find the easiest route to travel and you may have arcing across wires and to any ground points they touch.

I struggled with this for years until I went back to $2 copper plugs (from platinum) and set the gap back to .032" from .060". Nology and new Berus would arc with the big gap. I have now gone with Clewitt wires but have not tested them with the larger gap.

[Bob Kontak]

MSD recommends taking the gap up 5 thousandths at a time and test performance. They also recommend a starting point of 50-60 thousandths for cars with under 10.5 compression.

Maybe vash's 40-45 thousandths would not trigger the arching like 60 did in my application.

[JP911]

Bob - With the stock plug gap you're leaving some of the benefits of the MSD behind. To each there own.

[Bob Kontak]

No argument. I should have worded in a way that simply points out the risks vs. a disagreement. Sorry.

[walt83sc]

Good advice, but my concern is that the problem is digital, i.e. either on or off. At exactly 3400 RPM, the issue occurs. It runs fine up to that point, which leads me away from an "adjustment" solution. If something were out of adjustment, I would expect a gradual deterioration, not an immediate failure. It's almost as if the MSD were hitting the rev limit prematurely. I may experiment with the pills to see if it is halving the indicated RPM.

[dman]

I recently spent roughly 2 years chasing similar issues with a very similar set up, MSD 6 AL/blaster coil on an 83 SC (didn't need the tach adapter). Rough/no starting, engine cutting out at high rpm and backfiring.

Very long and expensive, multi ignition, multi shop story short, my standard blaster coil was going bad. I put a new high vibe MSD coil in and it runs perfect. You may have spark but it needs to be strong or you can get all kinds of strange issues. .