'75 CIS - unmetered air - what am I missing?

[fanaudical]

I've been chasing a problem (popping from the exhaust during decel) for months. I'm about convinced that I have an unmetered air leak somewhere in my CIS - and I can't find it. Here's what I've done (not at all in order until the end - everything done with the CIS assembled to the engine in the car):

- Several smoke tests of the intake and airbox - no detectable leaks. I looked at everything - rubber sleeves, injector o-rings and bushings, all the fittings, etc.

- Smoke tested the exhaust - found a pin-hole leak in the muffler, patched it, verified no further leaks

- Checked every single vacuum line - no leaks.

- Checked WUR - control pressures, system pressure - all fine. Checked vacuum actuation on WUR - no leaks.

- Checked decel valve. No leaks on vacuum actuator. No leaks across the valve when closed. Check all the lines to/from the throttle body and WUR for leaks - none found. Verified decel valve opens at ~20in-Hg.

- Tested injectors for flow volume, spray pattern, and cut-off. They all have good spray pattern and all flow the same amount. I did multiple flow tests at multiple flow sensor positions and found consistent delivery on all tests. They all shut off with flow sensor in the down position.

- (No auxiliary air regulator or enrichment valve on this car. EGR gone / plugged.)

- Verified the boot between the air flow meter and the throttle body has no holes and seals tight on both ends.

- Verified ignition timing, advance, retard, distributor play, etc - no issues and seems to be working correctly.

The one thing that's bugging me: I did find a very slow leak in the retard vacuum actuator. I plugged the vacuum line and still have the popping. Decided to run it that way a while and went to adjust the idle speed down (now that I have zero retard at idle). I was able to screw the idle adjustment screw all the way to the minimum stop (closed?) and idle speed was still a bit high.

This is leading me to think that I still have a leak somewhere in the intake - and just can't find it. Here's what I've theorized on possibilities:

- Airbox leaking somewhere that I can't see smoke from the smoke test?

- Worn bearings/bushings on the throttle body? (should have seen this on the smoke test...)

- Throttle body to airbox gasket (should have seen this on the smoke test...)

- Some other "bypass leak" in the throttle body?

I think the next step is to pull the CIS off the car and check every component the hard way. Before I do that - what else am I missing?

Thanks for the help.

[theiceman]

maybe im wrong but from my motorcyle days i found popping in the exhaust to be caused by rich running . un burnt fuel detonating in the exhaust.
if this is the case you could be looking at a leaky injector or struggling fuel distributor.
did you check injectors for leaking cut, off and spray pattern ?

mine pops through the exhaust on decel with my 74 sport muffler and SSIs .. sort or a burble, i just thought it was kinda normal.

[fanaudical]

Thanks for the suggestion. I guess I need to figure out some way to determine if I am truly running lean or rich short of adding an AFR gauge.

Sorry - forgot to mention - I did test all the injectors too. They all have good spray pattern and all flow the same amount. I did multiple flow tests at multiple flow sensor positions and found consistent delivery on all tests. They all shut off with flow sensor in the down position. Will add that to the list above.

I also have SSI's. It's past a burble - it's a pop. The car didn't do this until earlier this year.

[pampadori]

I always learned that the popping on decel on motorcycles after they installed better flowing pipes was due to the low speed jets being too lean. Never heard it mentioned that it was from being too rich.

When my car was CIS it started to pop more on decel for a few weeks before it got worse and worse and then finally had idle high issues and a rough idle too. That's when I found the airbox cracked.
I think you're on the right track personally.

[eastbay]

Why not add an AFR? That's crazy talk, shooting in the dark like that.

[75 911s]

Just a point of data. I fought my CIS for years. I refreshed all the rubber, swapped out the injectors, inspected everything, new seals. Each time I did these things, it would run good for a while and then start leaking from somewhere new. The pop off valve was the latest issue. i had that resealed and it re-tuned and timed and it ran great for a few months. Then it started to dog down again. I found out my last rebuild on the distributor was shoddy. It wouldn't keep timing and started running suspect again. Point is, there are many potential areas for problems with CIS. It can run good, but after 45 years they require a fair amount of continual maintenance. A lot of differed maintenance on a lot of CIS systems that I see too. That means a full refresh or you face what you are facing.

I dumped (probably at least) 2k into my cis fighting it. Wish I would have cut it loose from the get go and gone carbs like I'm doing now. I'm just ranting. CIS Is a pain. As for being helpful, if it were mine, I would take it off the motor and go through it again. ALso check your dizzy as the timing is super important to how the CIS runs.

[bkreigsr]

Originally Posted by fanaudical

I've been chasing a problem (popping from the exhaust during decel) for months. .....
.....I think the next step is to pull the CIS off the car and check every component the hard way. Before I do that - what else am I missing?.....

You neglected to mention how the car is running.
Other then the popping that's annoying you, is there anything else wrong with the performance?

[fanaudical]

Thanks for the ideas.

Little more history: I rebuilt the engine and CIS a few years ago (been on the road for ~ 7 years) and went through everything. Tony rebuilt my WUR, I flushed the fuel distributor, tank, lines, replaced all the CIS lines on the engine, etc. Rebuilt the distributor about the same time.

I have checked timing and timing stability. I haven't pulled out the oscilloscope lately to verify the trigger timing is stable but will do that.

AFR - Yes, I could add an AFR gauge and probably should. Was just trying to avoid doing that (really don't want to open up the interior again to add wiring, need a place to mount a gauge, take off the exhaust to weld in bungs, probably would want to deal with the old muffler while I'm in there...)

Ditching the CIS? I'll admit that I'm thinking about an EFI conversion. I have had a few cars with CIS (old Audi's before this 911) and have never found it difficult to maintain or keep running well.

The car is mostly running well other than the popping and some minor bucking at low RPM (<2400rpm) at light throttle (something else I interpret as a CIS lean condition). The popping is, to me, the big indication that something changed as it is consistent.

[Dave Kost]

What did you set the CO to? You really need a co meter to do it right. I think like Theiceman that you have unburnt fuel in the exhaust.

If it were me I would do a compression and leak down test. Re-adjust the valves.
Check if the cold start valve is working.( Not leaking)
Check it the fuel distributor is leaking at the piston into the airbox.

When you rebuilt the fuel distributor did you adjust the piston position in the airflow meter? When CIS Flowtech did mine they sent instruction.

Check for a misfire on a cylinder. Bad spark plug wire, cracked spark plug ceramic. What plugs are you running? Just coppers. I once had a cylinder go out after running for 20 minutes. it was a 912. Months to figure out. Here it was a resistor would short out when heated up.

Get a co meter or set the A/F ratio to the equivalent.

Sorry for the rambling....

[fanaudical]

No worries on rambling - all ideas are valid at this point.

I verified the CO was at 2.0% with a friend's Gunson Gastester this summer. I haven't moved the mixture since. Will try to get a CO reading again.

I did do a valve adjustment to see if it helped; no change. I did a compression test earlier in year; will re-do that and also find a leak-down tester.

Fuel distributor is not leaking. I have only flushed the fuel distributor - I have not had it apart. I did verify flow rate/pattern at various positions along the arm travel and didn't see any dips in flow rate. I guess it's possible the piston isn't in the right place, but why would that change all of a sudden? No indication of sensor arm slop/worn bearings.

Am currently running NGK copper plugs (new in December); will check those again. Plug wires test out fine.

[James Brown]

You can also use a smoke machine to help find the leak (or rule it out)

[Dave Kost]

Originally Posted by fanaudical

No worries on rambling - all ideas are valid at this point.

I verified the CO was at 2.0% with a friend's Gunson Gastester this summer. I haven't moved the mixture since. Will try to get a CO reading again.

I did do a valve adjustment to see if it helped; no change. I did a compression test earlier in year; will re-do that and also find a leak-down tester.

Fuel distributor is not leaking. I have only flushed the fuel distributor - I have not had it apart. I did verify flow rate/pattern at various positions along the arm travel and didn't see any dips in flow rate. I guess it's possible the piston isn't in the right place, but why would that change all of a sudden? No indication of sensor arm slop/worn bearings.

Am currently running NGK copper plugs (new in December); will check those again. Plug wires test out fine.

OK. running out of ideas but here goes

Intake manifold gasket leaking. Brake booster hose cracked.

How is the ignition distributor? Is it advancing ? Pertronix's or points?

Clean the ground connection of the CDI box. An old-timer told me he runs a dedicated ground from the CDI to the battery. Also, clean the engine ground strap. Lots of wacky running when the grounds are bad. Alternator ground too. You need a strong spark. Pull the coil wire and make sure no green slime oxidation is present. I use the red DEoxit cleaner.

You have probably done most of this stuff.

PS- I would try the CO at 3%

[7783911]

did you check your decel valve? that was my culprit

[stlrj]

Originally Posted by 7783911

did you check your decel valve? that was my culprit

Vacuum line likely got disconnected.

[930cabman]

Originally Posted by bkreigsr

You neglected to mention how the car is running.
Other then the popping that's annoying you, is there anything else wrong with the performance?

+1, Is she running ok? gas mileage ok? How bad can the popping be.

I would look for excess/unmetered fuel coming from an injector.

[fanaudical]

Thanks for the other ideas.

No brake booster on this car (and no vacuum hose). All other vacuum hoses are connected and verified as not leaking.

Electrical grounds are clean.

Ignition is advancing. I currently have a Pertronix ignitor installed. I do need to look at trigger stability (might be easier to just swap back to points).

Decel valve doesn't appear to leak. I will re-verify that it's opening at the correct vacuum level.

From above: The car is mostly running well other than the popping and some minor bucking at low RPM (<2400rpm) at light throttle (something else I interpret as a CIS lean condition). I will look at fuel consumption again (always a subjective thing given the wide use the car gets).