Please help me stop throwing away money to fix rough running 911!

[thejet95]

I recently bought a 1976 911 with the stock 2.7 motor in it. Took a risk had zero paperwork or history, but the engine ran very well until recently. Some weeks ago, noticed harder starting then usual and rough idling. Also sounds like it is “missing” when revved. So I thought a “tune up” was in order. I replaced the cap, rotor, plugs and wires. Still running the same. Bought a fuel pressure test kit and was unable to proceed because of multiple fuel leaks. The Nylon fuel line fittings had rust on them and apparently once I broke them loose, now leaked. So I bought all new fuel lines, banjo fittings, and while I was at it, new fuel injectors, sleeves, and o rings. Starting it up, and same rough running. So I started looking for air leaks. Did a smoke test and found zero leaks anywhere. Also replaced the fuel pump, fuel filter, and the fuel accumulator.

Did a re-test of the fuel pressures and here are my results.

System pressure. 4.5 bar.
Cold pressure. 1.5 bar (20psi) graph shows should be 1.7? WUR power disconnected.
Warm pressure 3.1 bar (45psi). With WUR plugged in.

Pressures seem pretty close to in range other then the cold pressure, but may even run worse when warm.

So I’m back to ignition. Re-wired some sloppy wiring around the distributor and installed a pertronix electronic ignition. Still runs rough.

Maybe a bad coil? I checked the resistance between the two posts and got 0.8, so I think that is in spec. Could the CDI cause this problem? Maybe I missed an air leak or 5? Haha. Also seems like its running rich, lots of smoke at startup and is “spitting” oily moisture out the tailpipe.

I’m stumped, low on funds, could use some help!

[EC900]

You done all the easy stuff. Could have a simple hidden vac leak. Tuning to and with a vac leak throws things up for grabs as I’ve found out, incl would be mixture. My vac leak came from under pedal cluster to brake booster, runs back thru tunnel hidden behind engine.
Seems the next step might be an eng. compression/leak down test to get idea on the overall performance. Condition of those old plugs would hold some clue as to type of fouling, as will new ones after a few more miles.
The smoke & oil substance would lead to believe time for a top end. How much smoke and how much oil spews out for how long (substance…water/moisture like bad gas or oil?)? Does smoking go away after few minutes. How’s the exterior leaks, possible head stud break or any noise from valve cover. On a well worn engine, rings and or valve guides has oil leaking into the cylinders is typical when engine sits for awhile.
What’s the mileage or rebuild history should be a clue, but the comp problem/leak test will verify.

[$yncro]

How's your timing?
I had almost the same prob m, turned out to be
timing issue.
Good luck.

[dlagerstrom]

Did you replace the green wire from distributor? If not, what does it look like?

Good luck.

[timmy2]

Originally Posted by dlagerstrom

Did you replace the green wire from distributor? If not, what does it look like?

Good luck.

No green wire from dizzy on a 76.
3 pin CDI and points.

[EC900]

Timing and dwell set after replacing those contacts?

[Swordfish II]

Could be a vacuum leak. This was the reason I went to megasquirt and cranktrigger ignition.

[thejet95]

Originally Posted by EC900

You done all the easy stuff. Could have a simple hidden vac leak. Tuning to and with a vac leak throws things up for grabs as I’ve found out, incl would be mixture. My vac leak came from under pedal cluster to brake booster, runs back thru tunnel hidden behind engine.
Seems the next step might be an eng. compression/leak down test to get idea on the overall performance. Condition of those old plugs would hold some clue as to type of fouling, as will new ones after a few more miles.
The smoke & oil substance would lead to believe time for a top end. How much smoke and how much oil spews out for how long (substance…water/moisture like bad gas or oil?)? Does smoking go away after few minutes. How’s the exterior leaks, possible head stud break or any noise from valve cover. On a well worn engine, rings and or valve guides has oil leaking into the cylinders is typical when engine sits for awhile.
What’s the mileage or rebuild history should be a clue, but the comp problem/leak test will verify.

Wow brake booster leak. I haven’t tried doing a leak down test or compression test. Partly because it was running so good for months then suddenly one day ran poorly. Only 38k miles according to the title and the odometer, but could be wrong. The smoke clears up after a few minutes. Definitely oily substance from the exhaust. Like it’s way too rich. Has a couple leaks under the car that weren’t there when I first bought it. Didn’t notice any valve cover noises, but I can check again.

[thejet95]

Originally Posted by $yncro

How's your timing?
I had almost the same prob m, turned out to be
timing issue.
Good luck.

I have checked the timing several times, it doesn’t seem to run better or worse at the z1 mark or -5 degrees. Not sure what else to check timing wise.

[thejet95]

It has the 3 pin cdi. I replaced cap, rotor, spark plugs and wires. Also installed a pentronix electronic ignition and removed the points. I attempted to adjust dwell but there doesn’t seem to be any adjustment with the pentronix. I thought about ITB and mega squirt too but would be too expensive right now.

[EC900]

My kind of in the same predicament w/my 77 2.7. Not too many 911 stock 2.7 MidYears in my locale or anywhere it seems, so comparisons and advice is best here.
The thing about my vac leak was that when i found the hose was hissing, replacing the booster hose altered the idle or Mixture that was set by PO. It actually ran better w/broken hose! I can only guess how many cracks in other hoses that are hidden in engine bay. See pic.
You can try a garage tweak with the FD mixture screw, 1 or 2 clicks counter-clockwise to lean it out. Where’s your idle set, 950 ? Some say older engine will run better a bit on the rich side. Pic of my tailpipe, no soot so I’m not set at that overly rich mixture. I’m waiting now until I can get into the shop to do some proper diagnostics and assessment. Oil seep is white smoke that one would see from worn valve guide, it was like a smoke house when I started up after 4 month hibernation…I’m at 200k, with last top end prob at 100k.
Surely check those plugs and gap again. Think there was some debate about using Petronix on the 2.7. Yes, No points/no dwell.
Being in storage kind of lends itself to lack of driving. I’m no daily driver so I’ll go out for a 30 mi highway, 5 mi stop/go city street jaunt at week once a week, always coming back running better than when I cold started.

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/31-CIS_Fuel_Injection/31-CIS_Fuel_Injection.htm

[Demetri]

How about the fuel? Is it fresh?

[RobFrost]

Oily exhaust at startup is oil leak running down into the heat exchangers. Run it for 5 minutes and it burns off.

Have you performed an Italian tuneup yet?

Sent from my SM-G988B using Tapatalk

[Zuffenwerker]

You really need to do a leak down test. After that pull your valve covers and have a good look in there and check your valve adjustment.
If your all clear after doing that try and determine if it’s fuel or ignition

[fanaudical]

Here are a few thoughts:

- Always verify ignition is working properly before diving into the CIS.

- You've mentioned plugs, wires, cap, rotor, and idle timing. Have you verified that the distribution is adding advance as it should with increasing RPM? These motors really run poorly without appropriate advance when revved; they don't seem to care much about idle timing.

- Oil sputtering out the exhaust - Have you checked the intake air box for excessive oil? Oil tank over-filled and ingesting oil through the breather hose?

- Definitely check for vacuum leaks. Pop-off valve fitted in the air box? Is it sealing? Got a hand-held vac pump? If so, check the line from the engine to the brake booster and verify that seals. Check vacuum retard on the distributor.

- Which WUR is fitted to the engine? Need that to help with CIS diagnostics.

- Leak-down test is a good idea if you don't find anything else. You described the problem as suddenly appearing; a leak-down test will pinpoint issues that took a while to get there...

[thejet95]

Originally Posted by EC900

My kind of in the same predicament w/my 77 2.7. Not too many 911 stock 2.7 MidYears in my locale or anywhere it seems, so comparisons and advice is best here.
The thing about my vac leak was that when i found the hose was hissing, replacing the booster hose altered the idle or Mixture that was set by PO. It actually ran better w/broken hose! I can only guess how many cracks in other hoses that are hidden in engine bay. See pic.
You can try a garage tweak with the FD mixture screw, 1 or 2 clicks counter-clockwise to lean it out. Where’s your idle set, 950 ? Some say older engine will run better a bit on the rich side. Pic of my tailpipe, no soot so I’m not set at that overly rich mixture. I’m waiting now until I can get into the shop to do some proper diagnostics and assessment. Oil seep is white smoke that one would see from worn valve guide, it was like a smoke house when I started up after 4 month hibernation…I’m at 200k, with last top end prob at 100k.
Surely check those plugs and gap again. Think there was some debate about using Petronix on the 2.7. Yes, No points/no dwell.
Being in storage kind of lends itself to lack of driving. I’m no daily driver so I’ll go out for a 30 mi highway, 5 mi stop/go city street jaunt at week once a week, always coming back running better than when I cold started.

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/31-CIS_Fuel_Injection/31-CIS_Fuel_Injection.htm

Yes idle is set around 950. My son lent me a wideband O2 sensor I thought I might try sticking it in the tailpipe and see what it reads. I think I saw a conversion chart somewhere for the wideband afr vs the CO meter.

[thejet95]

Originally Posted by fanaudical

Here are a few thoughts:

- Always verify ignition is working properly before diving into the CIS.

- You've mentioned plugs, wires, cap, rotor, and idle timing. Have you verified that the distribution is adding advance as it should with increasing RPM? These motors really run poorly without appropriate advance when revved; they don't seem to care much about idle timing.

- Oil sputtering out the exhaust - Have you checked the intake air box for excessive oil? Oil tank over-filled and ingesting oil through the breather hose?

- Definitely check for vacuum leaks. Pop-off valve fitted in the air box? Is it sealing? Got a hand-held vac pump? If so, check the line from the engine to the brake booster and verify that seals. Check vacuum retard on the distributor.

- Which WUR is fitted to the engine? Need that to help with CIS diagnostics.

- Leak-down test is a good idea if you don't find anything else. You described the problem as suddenly appearing; a leak-down test will pinpoint issues that took a while to get there...

Interesting you mention timing advance, the vacuum line was capped off from the advance when I bought it but it was running great so I didn’t change it initially. However in my quest to fix the rough idle I did re connect the vacuum but not sure it’s doing anything. I used a vacuum hand pump with the cap off the distributor and it does move the arm inside. Is there a way to test it with engine running? It has a pop up valve that seems functional, my smoke test didn’t show any leaks anywhere. I did overfilll the oil when I first bought the car and had oil in the intake box,talk about smoke I grabbed my fire extinguisher! I cleaned it out really well and the smoke dissipated. It was still running great for weeks after that. I will try and get a leak down test done, and let me see what my WUR model is.

[thejet95]

This is the control pressure diagram I used. My WUR is 0438.140.033

[fanaudical]

That's probably vacuum retard for idle emissions, not advance.

Hook up a timing light and run the engine. Observe timing mark. Disconnect vacuum hose to the distributor and check again to see if timing moved. Reconnect hose and increase engine rpm, make sure timing advances.

[RetroSC]

You mention it seems to run rough even when revved higher? If so, that's not usually how a new vacuum leak presents itself. The problem usually plagues idle the most and eases as the throttle increases. I'm feeling like more of a fuel or ignition problem here.

It's possible the car was not driven much previously so the fuel tank might have a lot of sediment in it. You driving has caused the filter or something else to clog a bit. Cold pressure looked a little low, but check pressures when revved up to make sure they don't drop.