Problem with CDI / ignition

[Tveder]

Hi All
Im new to this forum.... Im hoping for a bit of help and sparring on a problem I have with my 911 SC 1979 3,0.

I have been through a hole bunch of stuff - but have now narrowed the issue down to the following:

The car looses its ignition when the starter motor is running. And regains it at soon as you let go of the ignition key. I can start the car this way - just on the last spins of the motor - as it regains ignition when the starter motor is shut off.

To isolate the problem - and rule out any bad earth-wires or connections - I have disconneted the CDI (+ and -) from the harness, and given them direct power from the batteri. This did not help. It will not fire - even when the batteri is directly linked.

Then: I did a ton of checks on the ignition system including the CDI, coil, rotor etc. All seems good.

Then: I checked the voltage drop when the starter motor cracks. Checked it both directly at the battery, and at the CDI. Same result - it drops a bit more than I would like. From 12,3v to about 9,5-10v when cranking.

Then: I connected a second battery to the CDI, and disconnected the (+ and -) from the 6 pin connector. In this way I now have 12v on the CDI without any influence from the starter.
The car then fires right up... and runs fine.

So in conclusion i have a problem where I loose the ignition because my voltage drops a bit when cranking - taking the ignition out completely as the CDI will not work below 12v. I find that a bit strange because:

1. It could be the battery…. but it cranks the engine fine. I tried another batteri from another car - no difference. They could both be crap… but very unlikely.

2. The CDI... apparently requires 12v, and a small drop when cranking takes it out. Thats a bit strange i think… When i google it (its a perma tune 6 pin) its should work down to 5v.... However mine is an old one - and this might only apply to newer models??

3. The starter could be faulty, and pull way to many amps. However it cranks the engine just fine/fast, and sounds normal.

But definately the problems is here because of the voltage drop... the question is if its the CDI thats faulty and should be able to run at lower voltage?? Or is it the voltage drop itself thats the problem - and maybe my starter is blown??

Thomas / Denmark