Electrical Gremlin Back!

[mark 74 carrera]

I have a 74 911 w/ webers. I recently had a major mileage tune up. The regulator was replaced as a part my original electrical problems several weeks ago. The battery is new.

I drove the car to work this a.m.(about 50 miles). The car was at operating temprature, about 205. After a long stretch of interstate driving, 60 mph in 5th gear, the car instantly turned off. No sputter, no cough, just shut down.

I coasted with the car in fifth for about 5 seconds. Suddenly it came to life again. This was followed by the loudest back fire I have ever heard come out of my car. Also included was a huge puff of black smoke out of the exhaust. I drove the rest of the way to work without incident. It ran strong.

It seems to me that fuel is not the problem. (black smoke and back fire). The ignition switch was replaced about a year ago and is likely not the problem. I replaced the CDI unit about 1 1/2 years ago and I don't think that is it. Points were relpaced with the major service two weeks ago.

One more thing, for some period of time, my alternator was sending 20 volts back to the battery. (prior to battery and regulator being replaced) Could the coil have been previously damaged by the 20 volts prior to the regulator being replaced?

Any thoughts would be helpful.

[RoninLB]

non pro here, but the recent tune up leads me to the possibility that the dizzy external wire on the dizzy case on the engine side is loose..and a bad coil is always a possibility....Ron

[Zendalar]

Bad coil, loose ground/power to CDI, or bad CDI.

[mark 74 carrera]

Is there a way to test a coil?

[Zendalar]

You could test the coil with multimeter, I recall that I there is some info on ignition troubleshooting on the DIY section.

BUT, VOM uses so low voltage that it might not tell you anything, if high voltage is jumping through the insulation in coil, this can't be seen in VOM.

My legendary thing to blame is the CDI =) It might have a loose solder joint inside you should least check. Open the thing up, plug power to it and start moving/tapping those wires/parts inside the CDI with screwdriver (dont slip the screwdriver to metal parts).

Did everything else check out?

[jeff m]

Hi there,
Secondary of coil should be about 450 ohms, primary about 1 ohm, but it will probably be ok when you test it!
If you can get it into the failed state, stick a timing light on output of coil (or any plug lead).
Your CDI should be sending 300vdc to primary of coil, I use a 600v neon to check that.
If you are not confident with electrical testing then change the coil, because a faulty coil can destroy your CDI, and it's good to have a spare anyway.
If your fault persists with a new coil, then your CDI may be on it's way out.
But you haven't lost anything by getting a new coil, because if you need a new CDI later it's usual ( and safer) to change the coil at the same time.
Jeff

[stlrj]

With all that black smoke belching out your tailpipe, it might be a problem with too much fuel.
Are you running the CIS fuel pump? If so, it might be putting out too much fuel pressure for your Webers.

Cheers,

Joe