Hello all, My name is Aren and I recently bought a 1978 911 sc. The car ran great for awhile but one day when I drove home from work, just before I was planing to turn off the car the rpms started bouncing from 1-2k the engine was rev matching the rpm's. I took note, and about two hours later I went to figure it out. When the car started it bounced just the same as when I shut down however, it then died after 10-15 seconds of the rough running. The engine would not start again.

This is what I have tried so far:
pulled a spark plug and checked the spark, it was pretty week orange color and after two cranks it would not spark at all. To save time I just went ahead and changed the plugs, wires, coil, cap and rotor. After rechecking the spark it was much better bright blue but it still only sparked a few times and then the engine would just crank with no spark. I figured it was how I had the plug grounded (by holding it against a valve cover stud).
So still the car will not start.

next I pulled the cold start valve and it had a nice cone spray pattern for about 2-3 seconds so my thermal time switch and cold start valve work, along with my fuel pump.

I was getting a bit paranoid so I drained all the oil through some cheese cloth but, did not find any metal or derbies. I also looked really close at the screen and cut the filter open and nothing. Other pointless stuff I did "just in case" was pull the left side timing belt cover off to be sure the chain did not get lose and skip a tooth or something, it was good and tight. so I serviced the engine back up and checked the compression; all good.

so pretty much it brings me back to the spark going out after 2 fires, anyone have any ideas before I spring for a new ignition? I did clean all the grounds I could find and like I said before the spark is very bright but only for about 2 fires.

If anyone has had a problem like this please let me know what you did

Thanks!

 

Definitely check all your grounds, after 30+ years they need some attention regardless. Then turn towards the CD box. Are you running the oem Bosch unit or a permatune? My advice is the stick with the Bosch unit as I have nothing but bad luck with Permatune...... long story. You can have it rebuilt or find a good used one on E-bay or bone yards like DC Automotive, Parts Heaven, Oklahoma Foreign etc...etc... good luck!

 

My car has a Permatune on it, a new one costs 460.00 for a 78 sc, the Bosch units are much more expensive and I do not even have a core for a rebuild. I am thinking about a msd box for half the price I have noticed a lot of racers use them and vouch for them on the forums.