[Walt Fricke]

Mac - my understanding is that the Lambda doesn't affect idle, only part throttle/cruise. Idle uses a default value for the frequency valve. Disconnecting the wide band sensor would guarantee this, however.

What you mention about what happened at the shop sounds wrong to me also.

Check for Gunson - I think they made a CO meter. Those may come up used now and then.

There is a much fancier older generation electronic system which was cutting edge in shops back when, check HC and CO both (don't know about NOX). A mechanic lent his to me once, but don't recall the brand. It was about the size of a super super thick laptop, not the huge machines on a cart with casters which lots of shops had back then. In my state lots of shops could do the required emissions tests, but then the state created dedicated test centers - those became available used, too, but you need space in your garage if you get one.

Since you have an A/F meter, why not try adjusting the idle (iterative process between the mixture screw and the idle bleed screw) to get a better warm idle - like keep nudging it until you get to 14.7 for want of a better number?, with the idle RPM adjustments to about what Porsche says it should be. Does it then surge? How does it then drive? You can adjust with the engine running (at idle) watching your meter if you can see it from back there (I can see mine).

If you have made sort of large changes in A/F, might be a good idea after driving a bit to do another plug check. After idling should give you an idea of how idle is doing. For actual driving the protocol (fairly easy at the track) is to drive at a pretty high RPM/load, then turn off the motor, clutch in, and coast (brakes OK here) to a stop. Then pull a plug and look at it. It won't have had time to change from what wide open throttle does to it, is the theory there. Might be hard to do using your garage on a residential street.

Spark advance also affects idle, but should be easy to set to old specs? You've changed your engine some, might that change what the advance does? Wouldn't think it is behind the larger issues, though.