To make a long story short: I installed an MSD igntion in my 1980 911SC. Everything worked fine at first but then after a while the car developed an occasional misfire at mid rpm. I replaced the plugs and the problem went away for a while, but returned. I replaced the coil and the problem went away for a while, then returned. Over the last 5 years I have replaced the plugs numerous times, replaced 2 coils, rebuilt the Webers a dozen times, replaced the plug wires twice and the cap and rotor twice. I've had it on a dyno twice with wide band o2 sensors and was told(by an expert from California who shall remain nameless) that the exhaust pipes were too big for Webers. Smaller and smaller pipes were put on and a new distributor installed-every one of these things seemed to clear up the problem, for a while. Now, the car made 285 hp at the crank, even with the misfire, but the o2 sensors showed it running at a very rich 10:1 ratio in the mid range. I finally broke down and replaced the MSD box with a new digital model, and the misfire remained-haunting me with it's occasional pop. After all that, what was the actual cause of all the problems? The two wires going from the distributor to the MSD box were reversed. This caused a weak spark so with new plugs, perfectly tuned carbs and the right weather conditions, the car ran fine. But as soon as conditions weren't perfect, the weak spark would stop firing a plug or two. The car now runs absolutely perfect, you would think it was a modern fuel injected engine except the throttle response is far better-I wonder what it will make on the dyno now?

 

what color wires are you talking about?

 

The factory Porsche wire is green with an inner and outer strand, the MSD wires are green and violet.

 

ok, the trigger wires - are you saying you had the msd green & purple wires swapped initially where they connect to the coaxial green Porsche cable?

 

Randy:

It sounds like the trigger wires were connected out of phase from each other and that results in a 12 degree ignition retard. One should always check this with a timing light at every MSD installation.

Not good for proper running & performance as that results in a weak spark and sub-optimal timing,...

 

12 degree?? That part, I don't get. But thanks.

 

"12 degree?? That part, I don't get. But thanks."

The variable reluctance (magnetic pickup) type of distributor produces an asymmetrical signal
for triggering, i.e. a very fast negative falling edge and a slow rising positive edge. If the signal leads
are reversed, the slow rising (now slow falling edge) causes the timing to be retarded more so
at the lower RPMs and at idle where the timing is set. As the RPMs increase, the timing doesn't
follow the normal advance curve because of this.

 

Thanks.

 

Loose:

Not to highjack your thread but can you tell me more about your experiences with switching to Webers from CIS?

I really don't like the CIS system with it's surging and stalling and my car is old enough to not need to comply with emissions so the Webers seem like a viable alternative.

 

Yes, that's exactly it. And according to my timing light, the timing was fine with the wires in the wrong direction-but the difference was dramatic when I switched to the correct way.





Sure. The switch to Webers was pretty easy. PMO has the fuel pressure regulator that makes hooking up the factory fuel lines easy. The Webers themselves were troublesome at first because they had dirt in them but once properly rebuilt and run with dual fuel filters, they have been great. The power and throttle response have been awesome. In Bruce Andersons book he says that with Webers and SSI heat exchangers a 911SC will pick up 30 hp on the dyno but it will feel like 50 because of the increased throttle response and I have to agree. Here's a video of my dyno run: http://www.youtube.com/watch?v=hR-b4pbEw0U With a stock engine except for Webers and MSD ignition, the car did 0-60 mph in 5.6 seconds. I didn't dyno it at that time but once I added headers I put it on a dyno and it got 207 hp at the wheels, with a misfire. Then I pulled the engine apart and added 964 camshafts and pulled 247 hp at the wheels-with a bad misfire. Now that it's not misfiring, I can't wait to put it on the dyno again. If I were to do it all over again, I would go straight to the PMO carbs-everybody says they have all the advantages with none of the problems of Webers.

 

wait, I did the same thing and you didn't notice the ****ty running conditions? The thing should of ran like crap. Really weak, and it shakes and rattles oddly. your's didn't? That is really weird.

 

It would idle fine (when the plugs were new, carbs rebuilt and cap and rotor replaced) and would even seem fine above 5000 rpm, but everything in between was terrible. On the dyno I installed a wide band o2 sensor on both side of the engine and made sure the carbs were equalized and at idle to air/fuel mixture was 15:1 until about 3000 rpm when it would go to 10:1 and start coughing and sputtering then around 5000 rpm it would start to clear it's throat and stayed at 14:1 to redline,where it made 247hp at the wheels. The audio on the video I posted isn't quite good enough but you might be able to hear it on there. Now the car starts/idles perfectly and revs out smooth and clean so I'm anxious to put it back on the dyno.