Intermittent Problem - Loss of Spark??

[chris belyea]

Hi all,

I’m hoping that someone has experienced the problem that I had the other day and might be able to help me narrow it down problem on my 70 911E

I was out driving around town the other day for about 30 minutes. The weather was cool so it took some time to warm the car up. I took it out on the expressway so I could get the RPMs up. After about 5 minutes, I lost power. I looked at the tach and it wasn’t reading. The cluch was still engaged and after a few seconds later, it came back to life and I accelerated again thinking it was just a glitch. A few moments later, lost power again, pushed in the clutch and coasted to a safe place on the side of the road. I tried to crank the car, and it would turn over but didn’t start. I tired several times and then popped the deck lid to see if I could see anything. All the wires seemed to be securely attached. I tried a few more times to start it but it wouldn’t. Just as I was about to call a tow truck, I tried one more time. It started! So I drove off and about a 1/4 mile down the road it did it again. As I coasted looking for a place to pull over, I engaged the clutch and the car came back to life. I decided to not push the car and keep the RPMs below 4,000 and was able to make it back home without loosing power again.

I suspect that its loss of spark since the tach stopped working and not fuel (MFI). I have a suspicion that it’s related to high RPMs. The car ran fine around town and when I kept the RPMs relatively low. It seemed to fail when I would take the RPMs above 5,000. Any thoughts on what might be failing?

Thanks

Chris

[tom1394racing]

I had a very similar problem that turned out to be the CDI unit. It was a newly rebuilt Parts Klassic unit. I turned to Bob Ashlock for a replacement.

[steve shea]

Me too. A bogus PK CDI sidelined me. Ashlock fixed it.

[SFE]

Do you have your transducer (speed switch) in your car ?
If so you should try to disconnect it and see if it's better.

[chris belyea]

Thanks. My engine was recently rebuilt about 3 years ago. The engine builder sent me my CD box. Not sure if it was refurbished or new so I don't really know the history of the box so I'll have to look into that.

[chris belyea]

I do have a speed switch. I had it tested and refurbished a few years ago so I think it's in good order but I'll give it a try. When you say disconnect the speed switch, are you saying to remove the transducer itself (or perhaps the fuse that controls it)? Or merely disconnecting the switch at the throttle?

Thanks

[Rick Cabell]

Change out CD box with a known good one.

[sithot]

I smoked 2 PK coils. One created a fine towing bill as I was in BF Egypt when it happened. Both shorted to the case through the potting material. After the second time I used a cheap ass Pertronix coil on the car to get home and it ran fine. Did get another replacement PK coil which worked fine.

Now neither car has standard" coils. One has Richard Clewett's crank fire and the other car's engine jumped out and a 3.4 Carrera stole its place along with a nice fresh 915 with Guard LSD. The torque is fun.

FWIW Mr. Cabell had rebuilt the second (SC) engine a good while ago and it was running just dandy but an opportunity arose that I couldn't walk away from. It may get freshened and put back or go in another car. I like toying with "musical (chairs) engine" ideas.

BTW: Rick does very nice work if anyone has doubts.

[SFE]

I do have a speed switch. I had it tested and refurbished a few years ago so I think it's in good order but I'll give it a try. When you say disconnect the speed switch, are you saying to remove the transducer itself (or perhaps the fuse that controls it)? Or merely disconnecting the switch at the throttle?

Thanks

Just disconnect one (out of 2) wire which are connected at the microswitch !

[greens]

Check your coil wire. If it has a resistor, that could have failed. Happened to me