911SC: Jerks when cruising

[KoenigsTiger]

Hello everyone!

Finally getting to drive the 911 after a long rebuild!!!

It runs great, pulls even better. But once warmed up it jerks/hesitates a bit when cruising at a constant rpm... When I apply throttle or drop it there is no hesitation. Pulls great on acceleration. And drops off fine when throttle is released.

Its a CIS.

I did convert it to a 3.2SS from a 3.0... But I didn't adjust fuel or Air for the displacement increase.

What should I be adjusting

Thank you very much!!!
Gene

[nineball]

sorry i can't be any help but i had something completely different in mind when i read the title...

[flat6pilot]

It's possible that it's running (or going in and out of) lean during cruise.

You could take it to a good dyno shop and have a look at your AFR's while it's tuned. It'll cost you a couple $100 but might be worth it.

[KoenigsTiger]

nineball,

Yeah, I see what you mean. Too bad I cant edit the title of the post anymore... hesitates would be a better term.


Flat6pilot,

Yeah, seem like I could use some pro tuning.

Thanks a lot guys!

[Gunter]

SS 3.2 with non-Lambda CIS, 964 cams here and I set the mixture by ear, very easy to do.

What year/type is your CIS?

With or without Lambda?

What cams?
Overlap set at?

Ignition distributor running with vacuum advance only?
Or?
How do you set ignition timing?

[KoenigsTiger]

It's a Lambda CIS I think.

Its 1982 CIS. Its more of a PIA, right?

964 cams.

1.4 overlap, if I remember correctly

I still have the stock vacuum distributor. With both lines connected.

When I had the retard line removed and plugged, was almost non-drivable when cold.

Thank you Gunter!

[mede8er]

I'd switch my fuel pump relay out for the spare I keep in my glove box and see if that eliminates the problem....

[KoenigsTiger]

Brodie,

Yeah, I got new spare fuel pump relay. But there is no hesitation when I floor the throttle... So I guess the fuel pump pumps?

I'll give it a shot anyway...

[sjf911]

I don't think your fuel pump is the issue. Lean misfire, excessive vacuum advance, or lambda surging would be highest on my list to investigate.

[kramer747]

I thought some jerks were messing with you while you were cruising

[KoenigsTiger]

Lol Yeah, I see what you mean. Too bad I cant edit the title of the post anymore... hesitates would be a better term.


sjf911, yep I got to start investigating...

Thank you guys!

[Targalid]

With the change to 3.2 from 3.0 you have more airflow through the engine so I would expect a leaner mixture with the same fuel settings as with the smaller displacement engine. There are some good posts on this forum on how to set your CIS richer by ear, as Gunter suggests. I just did it on my 2.7 which was tuned for California with a very lean mixture and symptoms you describe. It took nearly three full turns of the mixture screw before it began to stumble because of too rich mixture which shows that I was risking burning up my engine from being too lean. If you don't have the right Allen wrench, Harbor Freight sells a nice metric set with plastic handles that are just the right length for this task. All you need for the CIS mixture screw is the 3mm. If you live in a smog control area you will need to follow up your adjustment with a CO check, though it looks like you may not. Try this description. It is the one I followed: How to setup CIS with no gauges It helps to read up on how the CIS works so you understand what you are doing.

[Joe Bob]

An afternoon with an exhaust analyzer would be a good thing.....

[Gunter]

Originally Posted by KoenigsTiger

It's a Lambda CIS I think.

Its 1982 CIS. Its more of a PIA, right? Yes, but Lambda works if all components are good

'82 CIS would have Lambda in the US but a Euro would be non-Lambda.
Is the OX sensor connected?
Do you have a CAT converter?
Is the system pressure and Control pressure cold and warm within specs?

964 cams. 1.4 overlap, if I remember correctly. Good. What is the CR and what octane are you using?

I still have the stock vacuum distributor. With both lines connected.

Was the distributor serviced-cleaned-oiled to be sure that mechanical and vacuum advance are working?
How exactly do you set timing?

When I had the retard line removed and plugged, was almost non-drivable when cold.
Strange because the Lambda CIS works really well with OX sensor connected AND the vac retard plugged and disconnected

Thank you Gunter!

I need more info to visualize your set-up. Are you, or someone else, doing the adjustments?

I suspect a mixture issue and timing issue that can be fixed. Learn how to set mixture by ear with the long 3 mm Allen. Correct mixture is obtained by small turns of the small Allen screw and the large slotted idle screw on the TB.
Set the ignition timing to the basic 5 deg BTDC and "play" with the mixture adjustment and idle. To find the right setting, turn the 3 mm Allen key a little cw until the idle gets rough (rich), then turn it ccw until it gets rough again (lean). Now go to the middle of the range and remove the key. Stop the engine. Make a white mark on the pulley 30 mm to the right of Z1. That will be the mark for 30 deg BTDC. Connect the timing light to plug wire #1 and open the lock-nut for the distributor. Start the engine and have someone keep it at 5000 RPM. Rotate the distributor to get 30 deg BTDC at 5k RPM. Watch out for the fan belt!
Stop engine. Tighten the nut. Plug and remove the retard vacuum. Start engine and re-set the mixture and idle ~900 RPM.

Please, answer all of the above questions.

[schumicat]

when was fuel filter last changed?

[KoenigsTiger]

Gunter, thank you very much for the help!!!

Its a US '82. Lambda is the O2 sensor, right?

I changed the exhaust. I didn't drive the car with the stock exhaust to notice the hesitation.

I have Euro Headers with an M&K 2in 2 out

The O2 sensor is on the Street adapter. I had to extend the wire for it. I crimped it... I wonder if the gauge + crimp splices played a role. But it works of voltage not resistant, so the voltage drop would be pretty small?

No CAT with my set up right now.

"Is the system pressure and Control pressure cold and warm within specs?"
you're talking about the fuel pressure right??? I haven't checked any of that...

964 cams. 1.4 overlap, if I remember correctly. Good. What is the CR and what octane are you using?

The overlap could have been 1.6... I can remember 100% I have to ask the mechanic, that helped me with the engine. He always used the same for all 964 cams... I went with it. I'll have to double check.

CR is 9.8 as per Max Moritz "spec." I never actually measured the at time of assembly... I had Shell 92(no ethanol) now 94 Husky(10% ethanol). same hesitation...

The Distributor "was, rebuilt" I was told when I bought the core engine. But I never checked my self...

To be honest, the last time I set the timing with the retard line disconnected. I haven't reset it for both vacuum lines connected... I got to do that first. Currently at cold the idle is fine, when warm its 800+-...

When cold it was barely drivable(with retard line removed & plugged), that was with stock exhaust, with the old 02 sensor.

I am going to ask the mechanic that helped me to adjust it. But I have a feeling I will be the one adjusting it...

Its a new 02 sensor, the fuel filter is new, accumulator is old. The fuel pump is old. But under throttle application or WOT throttle there is no hesitation...

Thank you very much guys!!!

[Gunter]

If there is no CAT, the OX sensor isn't going to do much, IMHO.

The curve of the distributor should be checked, IMHO.

Mixture definitally needs to be adjusted, I'd stay slightly rich.

Lambda will work fine if the Frequency Valve has the right duty-cycle.

Sounds to me that with a little tweeking and adjusting this engine should work fine.

[KoenigsTiger]

Gunter,

Can I just unplug the 02 sensor? And see how that works? Is there anything else I have to adjust for that?

Thank you,
Gene

[MBAtarga]

Unplug the O2 sensor and see what impact it has.

[snbush67]

It sounds like the interruption is a result of ignition, or fuel. You should consider what components of your CIS control fuel or ignition when at certain operating temperature and test them for correct operation.

I am not saying that you are experiencing detonation but here is some good information for tuning reference:

Detonation in a 3.2SS

Note the discussion about AFR setting at higher RPMs, and fuel delivery capabilities/limitations of CIS.

Or you could have bad gas, clogged injectors, fuel filter lines,etc.