Run a compression check on the engine, maybe a leak down. If the compression / leakdown test comes back okay, then don't worry about it. I've seen this before on rebuilt engines - sometimes unexplainable. All cylinders have good compression, but one is darker than the rest? Maybe some assembly lube burned in there? Dunno. It's perfectly normal for the pistons, combustion chamber, and exhaust ports to have carbon buildup.

If your compression test comes back with problems, then you need to decide whether or not to keep driving the engine, or to address the problem. I suspect you will have no problems, but if you do, then the prospect of a valve burning and falling off, is a distinct possibility. Then again, you may just tempt fate and the thing may last another 100,000 miles. Or only 100. Who knows.

Run the test - I think it will be fine. In reality, I would wonder the opposite - why the other four are so clean? They may actually be running too lean... :)

-Wayne

thanks all, I appreciate the feedback.

I'll get back to you all with some photos and some compression results, ASAP.

It's ok if you don't know the various people on this bbs. There are quite a few experts on 911s as well as people with expertise in other related areas. JW is one but not the only one. You can sort the expertise into about 4 groups, which overlap: experienced mechanics, techs, shop owners, experienced amateurs, race car builders, other expertise from specialists in ME, etc.