1980 SC starting challenge

[jass]

I have a recently rebuilt 1980 911 SC that I'm having a starting issue with. The problem is that the first time you turn the key to start it cranks fine but won't start. If you shut the key off and then turn again to start it starts right up - consistently!
Any thoughts what might be causing this starting issue?

[oneblueyedog]

Sticky fuel plunger? Cold start valve? Cold start valve sensor? Wiring to the cold start sensor (on chain cover near cly 1)? Fuel accumulator? Fuel pump check valve.

Try adding some Techron when you get down to an eighth of a tank. Run it around near the gas station. Fill up. Start checking the cheap stuff first if problem is not solved.

[boyt911sc]

Have you checked your fuel pressures? How's your residual pressure? This is a typical symptom of low residual pressure in the fuel lines during initial start. My car would have similar problem after sitting for weeks.

TD

[Gunter]

jass:
Make sure that the second yellow wire on the starter solenoid is connected. That wire goes to the timer for the cold start valve.
If there is still some air in the system, try this:
Remove the air filter, With ignition ON, Reach in there and lift the air sensor plate, You should hear the injectors hissing.

[jass]

Thanks for all the advice - I'll need to give these a go and see if I get any improvement.

Where is the Cold Start valve located?? Can the two wires on the cold start sensor be reversed - would that affect anything?

Also, this problem still happens when the car is completely warmed up - does te cold start sensor and valve stilll play a role at that time?

[oneblueyedog]

The Cold start valve is on the back of the airbox under the venturi. I don't think it has a polarity but the plug only fits one way.

If it doesn't start warmed up then it sounds electrical. I'd try switching the red relay with a known good one. Clean the connectors and make sure they are snug on the relay pins. You can spread the pins a bit with an Xacto Blade. Watch you hands.

Also check the sensor plate switch. Although when mine got funky, it would let the red relay run and would flood the cyls. The fix was a resetting of the sensor plate and complete PIA reset of the CIS.

[Gunter]

Jass:
There are many things that can go wrong with CIS.
In your case, I would suspect system pressure and/or control pressure especially the check valve on the pump.
To check the fuel pressure settings, you'll need special equipment and some knowledge. I suggest that you go to a Porsche-shop and have them check the pressure when cold and when hot.
I also suggest that you get the Bentley SC Repair Manual and read about CIS; there is way too much involved.
Long distance diagnosis is difficult to do. Guessing and replacing parts is not the answer, a systematic trouble-shooting will find the problem.
A proper tune-up will set the valve clearance, timing, mixture. They'll check for vacuum leaks, fuel pressure settings, etc.
Yes, it'll cost a few hundred Dollars but then you have a base-line.