911 Webers stumbling / lean at low rpm

[Alexandphoto]

Hi all,

Hoping to get some help here please as I’m banging my head against the wall with this one, just can’t figure it out. I’ve been having a bad run with this car and really want to get it right!! Feels like it’s so close.

I’ve been having issues with my 1976 911 with 2.7 motor running a Weber 40 IDA conversion. After the conversion, the car ran great. No issues with stumbling or leaning out

I’m now facing an issue currently where at low throttle (~20%) and low rpm (1000-2000) just above idle, my car is running lean on cylinders 4/5/6. Cylinders 1/2/3 are fine (I’ve got a wide and sensor in each header). AFRs and power are fine at full throttle, and AFR at idle (~12.5) is ok too..

I had the carbs professionally rebuilt about 2 years ago, but suspecting an idle circuit issue recently I rebuilt and cleaned them again. I also installed new Clewett spark plug leads

Afterwards, it ran great when I drove 15mins to my mechanic for my annual safety check (to renew registration in Australia)

It was running rich. Very rich, around 11 AFR at idle, but no stumbling or misfiring detected

As it idled in his driveway, it started misbehaving again. The exact same symptoms it had before, inconsistent idle and low load stumbling.

At WOT or higher throttle it is totally fine though, and AFRs look fine, even from lower rpm (2-3k)

I suspected it was fouling the plugs, so today I’ve leaned out the mixtures a bit which made no difference to the stumbling, but AFRs at idle are now around 12.5 which is a better spot.

I replaced the spark plugs with a slightly hotter plug (NGK BP7 replaced with BP6), and no difference at all. Brand new plugs too, so I don’t think they’d be fouling instantly with the new leaner mixture too. . With a synchrometer at idle there’s consistent readings between the side to side or throat to throat.

So now I’m thinking… what else could it be?

Float levels?

When the carbs were apart there appeared to be no issues with throttle plates, they were replaced when professionally rebuilt, no gasket leaks (I had the manifold surfaces machined and studs replaced too), and no fuel leaks since.

Sorry for the long message, I’m stumped and really need the help!

[rowingone]

If one side of the engine is running leaner than the other I would suspect the throttle arms need adjustment . . are you sure both carb sides come off their stops at the same time?

[Dpmulvan]

Check out the tech sections. Performance Oriented

[Chris78]

Originally Posted by rowingone

If one side of the engine is running leaner than the other I would suspect the throttle arms need adjustment . . are you sure both carb sides come off their stops at the same time?

This is also my experience. Have you checked side to side balance with syncrometer airflow meter at problematic rpm?

[buster73]

Could be an idle circuit issue (clogged/partially clogged idle jets...). Not unusual when carbs have been sitting for a while, especially with modern fuels (ethanol content). As already mentioned above, other culprits would be throttle linkage or balancing issue between left and right bank. Not a bad idea to perform a full carb tuning procedure (see the excellent „performance oriented“ link above). Worth to spend the time and enjoy a perfectly tuned engine afterwards.

[930cabman]

I would start by looking at your idle circuit. You stated she was running great, how long since the change? and what happened?

[lateapex911]

Spray a little carb cleaner or ether around the bases, and the throttle shafts....if you can at the offending RPMs. There should be no change in behavior. If it picks up RPM, then there's a leak. (I like to try easy stuff first )

The fact that it changed suddenly suggests that it has an issue that isn't related to side to side adjustment unless the adjuster/lock nut has somehow loosened, (IIRC that really can't happen?)
or ...some crud getting into the idle circuit is a distinct possibility.

Not sure what your ignition system is but if it's just one side that should eliminate a points or ignition issue.

[Alexandphoto]

Thanks all for the suggestions. I’ve had another crack at it today, and suspect it is in fact one of the carbs opening before the other.

I’ve rebuilt the offending carb a few weeks ago and thoroughly cleaned it out. I do not think that is currently the issue. Ignition wise, I’m still running the original dizzy, however points, spark plugs and spark plug leads(Clewett) have been replaced in the last 100km or so


I can see when I start to slowly rev the engine (~1500rpm) then one side has a leaner AFR which I suspect means it isn’t opening properly.

I’m thinking I’ll get a professional to try and align the linkages. I just can’t seem to get it right…

[Chris78]

Originally Posted by Alexandphoto

Thanks all for the suggestions. I’ve had another crack at it today, and suspect it is in fact one of the carbs opening before the other.

I’ve rebuilt the offending carb a few weeks ago and thoroughly cleaned it out. I do not think that is currently the issue. Ignition wise, I’m still running the original dizzy, however points, spark plugs and spark plug leads(Clewett) have been replaced in the last 100km or so


I can see when I start to slowly rev the engine (~1500rpm) then one side has a leaner AFR which I suspect means it isn’t opening properly.

I’m thinking I’ll get a professional to try and align the linkages. I just can’t seem to get it right…

Have you checked with an airflow meter?