Difficult cold start and surging

[Early911SC]

My 78 SC is stick with 41k mi a barn find purchased 10 years ago. Not driven often but been reliable until recently it will not start at all. Troubleshooting ignition confirms sparks so illuminating CDI. Just went I get ready to tow her to a shop, she starts but idle roughly while backfire when press on gas pedal. Once warms up she idle nicely with no backfire. Is this a WUR issue? How to approach further? Thanks!

[PeteKz]

Not starting: Basics first.
Compression (I assume that is good because the car runs after you get it started).
Spark: How did you check spark? Use a spare spark plug and plug it into one of the spark plug wires. Hold it against the engine. Crank the engine to make sure you get a consistent and bright spark.
Fuel: If it is hard to start, try squirting starting fluid into the intake. If it fires on starting fluid, then you know you don't have enough fuel for cold starting.

Based on your description, especially the backfiring, I suspect the engine is too lean (not enough fuel). It could be the WUR is out of adjustment, Or more likely you have intake air leaks. Look carefully around the front and back of the CIS system to see if anything is loose or hoses not connected. Make sure the cap on the oil tank is tight and the cap seal is in good condition.

CIS systems are very sensitive to air leaks.

First do the starting fluid test, and report your results. Then we will figure out the next steps.

[proporsche]

here is your quide to troubleshooting...
Ivan

[917_Langheck]

Backfire (exhaust) is too rich

Misfire (intake) is too lean (as in "lean misfire")

Proper description is important for diagnosis.

Either condition can be induced by improper fuel:air settings/leaks, or timing issues.

[Early911SC]

I start the engine again this morning after cold soaked overnight So Cal weather 55 degrees lol, she starts right up idling at 1200 about a minute into warmup I try to accelerate the engine gently but I notice a hesitation, then a loud pop. As Langheck suggested and clarify, I say it’s a misfire ie intake. I do have a pop off valve and it’s my second intake manifold since the original one was damaged by the misfire, I recognized the sound. Btw the shop installed the POV backward, however it still work somehow occasionally it would stuck open with the air filter screen. In such case, I won’t be able to start the engine while removing the air filter I can hear the valve pop back in place. For this exercise I removed the air filter. Once the engine warms up, she idle nicely and accelerate without any hesitation or misfires. I went thru a few of the troubleshooting threads and noticed the WUR is reliable however for a 45 years old car and likely original part, it may be the source of the problem. My prior 911S was driven for over 200k mi from 1986 to 2014 with one overhaul in between, but never had any starting issue so I’m new to CSV, WUR terminologies.
Prior to this several months ago, while warming up the engine just dies. Thought it may be the CDI but got sparks by removing a wire from the distributor cap creating 1/2 inch gap. Spray starting fluid into the intake with no respond. Changed the red relay with no respond. Changed the 5 years old battery to full size Interstate, cranking much faster but no start. Then after several week sitting outside, one final attempt and surprisingly she starts and runs roughly. Today she runs much better not sure it’s the fuel or got a clog in the WUR. Thanks all!

[sugarwood]

Originally Posted by PeteKz

Not starting: Basics first.
Spark: How did you check spark? Use a spare spark plug and plug it into one of the spark plug wires. Hold it against the engine. Crank the engine to make sure you get a consistent and bright spark.
.

No one ever explains this correctly.
No one.

How do you prevent the engine from starting while you're holding a spark plug in your hand? This seems dangerous as ****.

[Early911SC]

Since 78 SC required smog check, this SC is stock with no modification including original cat. I had to make a slight adjustment with a 3mm hex wrench with the help with the smog shop. The final reading as follows

1600 rpm CO2max 14.4 meas 1.2 HC 214 meas 37 CO 1.36 meas .24 NO 1364 meas 400
2500rpm COmax13.5 meas 2.3 HC 181 meas 19 CO 1.16 meas .16 NO 1224 meas 275

[Early911SC]

I check spark by removing a wire from the distributor cap with a 1/2 inch air gap between the cap and the wire. I set up my iPhone tape to the bumper and record while I would get inside to start the car. Upon reviewing the video, the sparks was there consistent with the cranking speed.

[Early911SC]

New to this but my earlier post is being approved by a moderator, thanks everyone!

[PeteKz]

Originally Posted by sugarwood

No one ever explains this correctly.
No one.

How do you prevent the engine from starting while you're holding a spark plug in your hand? This seems dangerous as ****.

Hah, hah! Well, if you hold the spark plug without touching it to ground, and it makes you shout "S%#$", the spark is good!

It doesn't matter if the engine runs while doing this test, but a better idea would be to get a spark tester and put that inline. Then you can run the engine and see the spark jump inside the tester section.

I know there are warnings on the CDI box, coil and other parts about high voltage dangers. I've been shocked by ignition systems so many times I can't count them. I haven't died yet. Not even once.