SC With starting a starting issue
 

So heres how it is. Just bought an 1980 911SC last summer. It's been a great car since then. But today when i went to go start it..Nothing. Hooked up one of those portable jumpers to it...lights went on and tried to turn it over with everything else turned off and it turned a couple small times but didn't have enough to get it going. Then had an Audi hook up to battery...Still not enough to get it going. So I had then had the audi going about 5000rpm and the Porsche turned on. But when I was backing out of the garage, it stalled out on itself. So again after going through the 5000rpm with the other car, I left it connected to the Audi for probably 10min. Disconnected and then took it out for a drive. Drove it probably 20 miles with no problems. Got back to the garage and turned it off. Then Tried to turn back on an got a weak turnover. The Battery measure 11.5 volts so it should have no issues starting. Any ideas on what might be causing the problems?:confused:

You could have high resistance in your starting circuit from poor/dirty connections or poor/dirty grounds. The fact you still got a weak turn over with the Audi connected could also mean there is high resistance somewhere--from the battery itself, or connections.

Before you do anything, check that all connections to the battery are clean and solid, and all the ground connections are clean and solid as well. Check the ground strap from the battery to the chassis. Check the ground strap on the transmission. Check the ground strap and connections at the starter. Make sure all are clean and tight.

If you still have a problem after cleaning/tightening all connections, the battery is still a likely suspect. How old is the battery? Just because a battery shows a certain voltage does not mean it is able to supply sufficient amperage under load. A dying battery often does not hold a charge long enough to supply the high demand for a starter. Have you tried turning on your head lights to see how bright they are and if they begin to dim within a short period? (I'd suggest the old trick of trying to start the car with the head lights on but I think they automatically switch off when the key is turned to start.) If you battery is of unknown age, take it to a parts store for testing under load. If all the above fails to remedy your problem, report back your results. You could have a mechanical problem such as a bad bushing at the starter that is causing your problem. You also may have an electrical draw that you are unaware of that is draining the battery. There are ways to test for that, if needed.

11.5 volts is a dead battery.

+1, Needs another volt to start. And a bunch more amps.
Pull it and have it tested. Then buy a new one...

Well thanks for help so far. I got it up and running yesterday. Took the battery out and cleaned up those battery connectors really well as well as the battery terminals. Then Jumped it with a portable jumper and it fired right up. After running it pretty hard, I had it fully charged. Another question though is that i read somewere that a loose ground connection could be causing other electronics to have issue. That being that the tachometer is jumping all over the place and i was wondering if this had anything to do with it. It only started doing this about 2 weeks ago and it never did it it the past 3/4 year that i have had it. Thanks