In need of enlightened viewpoints

[FredsRed911]

BACKSTORY
I wrench all my toys, have a lift in my garage and worked my way through undergraduate school in a junkyard. Bought car 18 years ago non-running. Got that resolved in short order w/used fuel distributor and replaced/checked most everything above block at that time along with fuel pump. 18 years pass, put two kids through college, paid for a wedding and now retired. Took car out, started bucking, shut off 4 times in 8 miles but would eventually restart and made it back home. Each time it ran for shorter intervals.

ACTIONS TAKEN
Given the years since serious time and monies spent and the age of car, I have just replaced, with no compunction:

Original tunnel lines and fuel lines with OEM spec tubing
Inspected tank condition
New Pierburg fuel pump
New Bosch fuel accumulator
New fuel filter
Rebuilt CDI box
New voltage regulator
Pulled fuel distributor and WUR and sent to CIS Flowtech in March
Bench tested fuel injectors
Cleaned and spec-ed fuel/air flow sensor plate and installed new gasket
New CSV
Pulled alternator and had it bench tested
New battery

Fuel pressure checked directly to FD port (its a '74 w/banjo fitting to Throttle Control Sensor that has to be removed) 67psi

fuel pressure checked w/banjo to Throttle Control Sensor set to spec at 30psi at idle, 37psi mid and full throttle 27psi

Fuel pressure to WUR is 44psi

Setup fuel distributor per 5 step process on this forum.

SYMPTOM:
The car is still starting (like an early CIS car) then runs well for 15-30 seconds and then it dies, literally just like turning off the key; even at 3000 rpms, no backfire, no wavering, just dead. All the above parts needed to be done as two Q-tips (65 year olds) standing on the side of the BRP in the Smokies "ain't gonna be a good look". But, I am running down to a very short list of possibilities and would appreciate viewpoints. Ignition switch, tach sensor.....metorite shower?? I will be running continuity checks on grounds and electrical leads this weekend, while awaiting your thoughts. This forum has been invaluable to me over the years!

[Balisong]

Lots of good suggestions in here

https://forums.pelicanparts.com/porsche-911-technical-forum/895178-1975-bucking-2200-rpm.html

[Dpmulvan]

Bad coil possibly

[FredsRed911]

Forgot to mention, replaced coil and 3 prong wiring harness from Parstklassic when CDI box was replaced....but I get where you're going

[47silver]

My 75. Would start easily. Then on the road shut off. Checked everything. Decided to replace the points with a pertronix. Found that the black points wire was rubbing in the distributor shaft. The wire was bare. When it would get hot it would touch the shaft and shut off the car.
Replaced the points and never had that issue again

[brianlay]

When it dies it won’t restart immediately? Then begin troubleshooting as a no-start condition.
Check for fuel, spark, etc

[E Sully]

Electric items all seem to be new, although that's not a guarantee. The suggestion to check the distributor and points is a good idea.
As far as the fuel system, the car sat a long time. You said you checked the tanks condition. I'm wondering if the filter screen in the bottom of the gas tank has ever been cleaned. Is it possible the fuel supply is restricted and only flowing enough for starting and short runs. One way to check is to open the return line to the tank and see if your getting 850cc in 30 seconds.

[PeteKz]

Well, Fred, since you put yourself through school working in a junkyard, I won't tell you how to suck eggs, BUT, I'll bet that most or all of the stuff you replaced is still good, so save it for spares and testing. Except the battery. After 18 years that was surely toast.

You know the drill: Compression, fuel, spark.

It sounds like the spark is shutting off. Does this car have a "pop-off valve" installed in the air box (prevents breaking the air box if you get a backfire)? If so, then first test is to spray starting fluid into the air filter intake and see if it immediately starts. If starting fluid makes no significant difference, then you can start down that branch of the troubleshooting tree. Make sure the CDI box is working--it makes a high-pitched squeak when power is on.

If it immediately fires with starting fluid, but does not immediately fire without it, then look for what's stopping fuel. Make sure the pump is running--you can hear it. You also checked for CIS pressures, so leave the gauge connected and see if the pressure drops when it dies.

When you start it, leave it running in the driveway until it dies, then try to find why it died.

One other thing to try: Pour a bottle of Chevron Techron Concentrate into the tank. I assume you filled it up with fresh fuel...

Post your results back here.

[boyt911sc]

Originally Posted by FredsRed911

Fuel pressure to WUR is 44psi

Fred,

What was this pressure? CCP OR WCP? Thanks.

Tony

[turbo23dog]

Hmmmm… fuel or spark problem, which one can it be? I had similar engine cut off problem and we found that the screw holding the wire to the fuse for the fuel pump was loose. At random times going into a corner or hitting a bump in the road, the wire would move and the fuel pump would cut off.

Good luck!

[FredsRed911]

47Silver's comments prompted me to order new dizy cap, rotor and points along with a coil to dizy lead wire to ensure total ignition refresh. If the individual plug wires were bad, it'd misfire and its not.

Brianlay's comments are correct in that it requires time between restarts. Pulled sparkplug and spark is present, plug appears wet and fuel/air plate has pressure. Can hear both the new fuel pump running and CDI box "squeaking".

E Sully & PeteKz, my apologies, I had been driving the car regularly for the last 17 years until the problem surfaced. I am going to check out the return volume of fuel upon that suggestion. I tend to hoard my used parts that when replaced make no difference in the solution to the problem.

I will squirt some ether into the airbox to see if that is the issue, have noted wet sparkplugs upon removal, and it does have a popoff valve. I will install the fuel pressure gauage and then run it as long as I can.

boyt911sc, the pressure was CCP. Guess I can remove WUR for the oven to check for hot, apologies for not specifying prior.

Will report back by mid-week, thank you all!

[RSTarga]

try a different CDI, who did the rebuild?

[997at]

If it starting and running just fine for a while and then randomly cutting out very abruptly as if the key were turned off I’m putting all my money on it being a spark issue.
Number one culprit would be the CDI. Best advice I can offer would be to junk it and install an MSD ignition. It could be a bad coil. That’s another good reason to go with MSD and the compatible coil.

[mysocal911]

Originally Posted by PeteKz

Well, Fred, since you put yourself through school working in a junkyard, I won't tell you how to suck eggs, BUT, I'll bet that most or all of the stuff you replaced is still good, so save it for spares and testing. Except the battery. After 18 years that was surely toast.

You know the drill: Compression, fuel, spark.

It sounds like the spark is shutting off. Does this car have a "pop-off valve" installed in the air box (prevents breaking the air box if you get a backfire)? If so, then first test is to spray starting fluid into the air filter intake and see if it immediately starts. If starting fluid makes no significant difference, then you can start down that branch of the troubleshooting tree. Make sure the CDI box is working--it makes a high-pitched squeak when power is on.

If it immediately fires with starting fluid, but does not immediately fire without it, then look for what's stopping fuel. Make sure the pump is running--you can hear it. You also checked for CIS pressures, so leave the gauge connected and see if the pressure drops when it dies.

When you start it, leave it running in the driveway until it dies, then try to find why it died.

One other thing to try: Pour a bottle of Chevron Techron Concentrate into the tank. I assume you filled it up with fresh fuel...

Post your results back here.

It's that simple! Less than a two minute effort, and you'll know where to focus your troubleshooting, i.e. spark or fuel.