my 1975 911 re-start problem

[cestlavie]

after driving my 911 for approx. 1/2 hour and then stopping for breakfast, it will not re-start unless off for 1.5 - 2 hours. It turns over fine, but won't fire up. Is there something that has become defective that has to cool off for such a longtime?

[75 911s]

Sounds like the Coil. Question did someone replace the black coil with a silver one? Silver coils are junk. If not the coil, another common issue is vapor lock. Usually you can still start it, but it's difficult.

Replace the check valve at the fuel pump. It's in the drivers wheel well area.

[clarkd]

Probably a long shot, but I had a points gap problem causing hot start issues.

[yelcab1]

75 is a CIS system, right? warm start problems have many causes. You need to do some troubleshooting first.

[Paulporsche]

Could be many possibilities, incl points, CDI box (is it a Permatune?), leaky injector, thermotime switch, check valve or fuel accumulator, mixture set too rich, WUR out of spec or not functioning, vacuum leaks, and more.

Where are you located? How hot was it when you drove? How hot did your oil temp gauge get? Any other start/run issues? When did you last do a tuneup?

[72911]

Everyone missed it. Welcome to the forum....sorry that your first post is for trouble shooting. You are in good hands here for that (and lots of other things too).

[fanaudical]

Welcome to the forum.

Here's a helpful link to a good CIS trouble-shooting site:

911 CIS Primer - Index

A '75 has the hand throttle (and I'm assuming it's adjusted correctly) and the fuel pump will run as soon as the key is in the "on" position. Something to try next time it won't start when warm:

- Hand throttle all the way down.

- Key to "on"; listen for the fuel pump and let it just run for ~30 seconds. The point of this is to try to prime the fuel system if the accumulator or check valve are bad.

- Pull the hand throttle up ~1/4" and engage the starter.

If the thing starts then it's most likely the accumulator or check valve on the fuel pump. If it doesn't, then next step is an ignition check (presence of spark). If no spark, check points, coil, CDI, etc. If you've got spark then it's time for a full CIS pressure check.

[Discseven]

Had that "had to wait to start" in '74 Carrera. Turned out to be warm up regulator (WUR). Not sure if '75 engine is same config.

[Paulporsche]

I had this happen recently on a very hot day to my 77, which has an 80 engine but still uses a breaker point distributor. I did two things: leaned my mixture slightly and reset my points. The problem went away the next time I used the car.

I subsequently switched to a Pertronix ignitor in place of the points, and replaced my sparkplug wires, distributor cap and rotor. I haven't driven the car much since I did this, but so far I haven't had a recurrence of the problem.

Why not look into the easy, obvious stuff first. Check your ignition components, including your point gap. If this doesn't work you can move on to some testing.

[viejopatron]

How old are your batteries? My '71 behaved the same way. New batteries fixed it.

[LlamaMan]

I had the exact same problem, took years to solve because could never be reproduced at a repair shop. If could be vapor lock if it's really hot, or a bad fuel accumulator, but sounds like you have ruled out vapor lock. In my case it turned out to be a flaky CDI unit. Eventually it was swapped with one from another car and it never happened again. Perhaps you can find someone or a shop willing to swap ?
Good luck and welcome to the forum.

[hughc]

cestlavie is a neighbour of mine and I'd like to be able to help him out with his hot start problem.

He called me out last week to help him when his car would not start after being shut down for 45 min or so. It did eventually start on it's own, not because of anything I/we did.
I got me to thinking that I had the exact problem back in '17 but I can't remember what was done to correct it although I'm leaning towards the WUR. I'll have to check my invoices to see if that time period coincides with when I asked Tony to recalibrate my WUR.

Since I now have a set of gauges I would be willing to check his system (cold) but I am not sure if the WUR pressures for a '75 are the same as for my '83.

Can anybody confirm this for me?
Thanks

[fanaudical]

WUR pressures are most likely not the same. Need to determine exactly which WUR is installed and then find the chart. Here's a link to a thread I started about troubleshooting my '75 with the 009 WUR and it has the chart in it:

Sorry - Yet another CIS troubleshooting thread...

Be sure to check pressure retention and warm control pressure too (with/without vacuum).

[Flat6pac]

If your able to start by removing the air cleaner and lifting the the horse shoe for a couple seconds with the key on, do the check valve on the fuel pump.
Bruce