[Richard Curtis]

this has been nagging at me all evening, after watchingyour little video of the tachometer jumping around...

Right after I bought my 964 six years ago, I had a similar situation; the car ran erratically, often shutting down. I barely got it home to my garage where I spent a lot of time scratching my head. Eventually, I cleaned all the ground connections, especially the ground connection between negative battery terminal and body. Problem solved.

You might also go back and double-check that the engine/trans ground strap is firmly attached and has clean connections.

[smdubovsky]

The MSD generates the signal to the tach. Looks busted. Put another or CDI back in.

[Vicegrip]

Originally Posted by smdubovsky

The MSD generates the signal to the tach. Looks busted. Put another or CDI back in.

+1 I don't understand the confusion here. You have a group of issues that can all stem from the MSD. You need to swap it out with a known good one. A malfunctioning MSD can cause spark timing shifts in addition to dropped spark pulses.

I have a CD box from a 77 if that will work for testing.

[VaSteve]

Originally Posted by Vicegrip

+1 I don't understand the confusion here. You have a group of issues that can all stem from the MSD. You need to swap it out with a known good one. A malfunctioning MSD can cause spark timing shifts in addition to dropped spark pulses.

I have a CD box from a 77 if that will work for testing.

Does the 77 have 3 or 6 pins? Do you have the matching coil? I'm sending both back for testing.

[Vicegrip]

3 pin. No "matching" coil but I have a 77 motor with a standard 2 wire coil on it coil on it.

[VaSteve]

Originally Posted by Vicegrip

3 pin. No "matching" coil but I have a 77 motor with a standard 2 wire coil on it coil on it.

I meant non-MSD coil since mine if going back for testing as well. Don't think a 3 pin would work on my SC....it uses 6. Thanks for the offer.

[smdubovsky]

SCs should use 6. I think I have some 6-pins. Should have a coil too but I'd have to dig for that to be sure. Though any CDI coil should work.

[OldTee]

Replace the green wire to the distributor.

ARF

[VaSteve]

Originally Posted by OldTee

Replace the green wire to the distributor.

ARF

Been done. Been in operation for several years as well.

[arob]

(Tracking this thread) - I encountered similar problem this past weekend at VIR. Warm up lap of the last session on Sunday, coming up the esses, my car just started running like shit, eventually cutting itself off at South Bend. I was able to coast down the hill, and park it next to the Oak Tree flag station.

When I got towed back into the paddock, I started it long enough to get it onto the trailer. Sounded like shit, RPMs jumping all over the place, and eventually cutting off. Riley took a listen to it for about 15 seconds, and thinks it's a vacuum leak.

Monday morning, I pulled the air box off and noticed that the pop-off valve had cracked off at one end of the spring hinge (non-fixable). Will be ordering a new one.

Fairfax Jim came over and we tested spark and fuel pump, which turned out fine. Started it back up again, and after about 10 seconds of smoke, it started running fine again. Took it around the block, and all seemed well.

I'm still not sure what went wrong (besides pop-off valve), and am not totally sure it fixed itself.

I've got 3 more events until the end of the season, and I'm hoping that it'll survive.