76 2.7 No Start

[Balisong]

Hello all, is there a good troubleshooting guide for 2.7 with CIS?

It’s a fresh rebuild with about 1500 miles on the rebuild.
Has always stumbled a bit for the first 10 seconds after starting it cold…then it runs perfectly. Takes a little longer than expected to start when hot.

Yesterday during the stumbling phase (while cold) it died. Now when I try to start it, it just turns over, tries to fire, but then dies. Keeping the starter engaged no help.

So far I’ve just checked fuses front and back. Made sure no frayed wires in engine bay or near distributor. Battery is new and strong and fuel is non ethanol premium and also new.

I’m not sure if it’s fuel or spark. Fuel pump is always dead silent so I can’t tell if it’s working just by sound.

Thank you!

[jma0506]

I'd recommend starting with this thread CIS Troubleshooting for Dummies

Sounds like you should verify your fuel system pressures as a starting point and go from there

[Schulisco]

I guess it's mixture is too rich through too low control pressure by the WUR. Check out the control pressure with a fuel pressure gauge, like https://jimsbasementworkshop.com/CIS/pages/test_pressures.html

These are the cold pressures from the '76&'77 engine:



Troubleshooting the CIS system:
https://jimsbasementworkshop.com/CIS/pages/test_troubleshoot.html

Kurt from KlassikATS has some very useful and informative videos on how to work on a 2,7 911 engine and on how to diagnose problems: https://www.youtube.com/channel/UCbo48hJkHDikGut5DoL6-sQ/search?query=cis
For more tech details refer to his webpage https://www.klassikats.com/

[T77911S]

remove the connector on the top rear of the AFM so the pump runs with key on.
verify it runs.
if you have gauges, check pressures and post.

chances are the system was "tweaked" over the years to make it run right.

best thing to do is when running you can move the sensor plate up or down by hand to determine if the mixture needs to richer or leaner. if it run better pushing up make it richer.

post fuel pressures.

[Balisong]

[jma0506]

Looks like you have a minor vacuum leak

[Schulisco]

Okaaaay, mixture is not too rich, it's too lean then ... I recommend a high quality 2 component glue which is also capable of heat (at least 212°F/100°C) and chemicals/fuels.... otherwise you'll have to fix this from time to time again....do not use some hot glue guns, this will get liquid again while heating up...I used this: https://petec.de/en/products/?prodid=150

Pop off valve on SC
https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/32-Pop_Off_Valve/32-Pop_Off_Valve.htm

Thomas

[boyt911sc]

Originally Posted by Balisong

Balisong,

For better sealing and adhesion, mount the POV’s shoulder/collar section against the flat section of the airbox instead on the ribs’ edges. So you need to trim the vertical ribs a few mm to allow the collar section to go down.

And test the POV before applying the epoxy. You could do a simple test on the POV by doing a leak or vacuum test. I have several POV’s new from the box that failed the tests.

Tony

[Balisong]

Back on the road. Thanks all!
Thank you Tony. When it wouldn’t start I was wishing you were there.

[Schulisco]

Originally Posted by boyt911sc

Balisong,

For better sealing and adhesion, mount the POV’s shoulder/collar section against the flat section of the airbox instead on the ribs’ edges. So you need to trim the vertical ribs a few mm to allow the collar section to go down.

And test the POV before applying the epoxy. You could do a simple test on the POV by doing a leak or vacuum test. I have several POV’s new from the box that failed the tests.

Tony

Correct, the better prepared and bigger the surface is cleaned and even, the better the sealing.

I also saw POV missing the rubber sealing / O-ring inside...this might be one reason, the other is that those rubber O rings won't seal properly. But this can also be negligible - when the engine is running, its generating enough vacuum to seal this anyway.

Thomas