Sick QV Cylinder - Any suggestions?

I have recently finished a complete motor overhaul on my 85 308 QV this fall, and am just ironing out the final issues with this troublesome motor. To make a really long story short, the motor was in really rough shape when I aquired it and there was no history. Engine was removed, torn down, and every inch of it was gone over to find and correct the many internal problems that lurked inside. I have driven the car about 300km since the motor rebuild, but was experiencing a performance issue where I knew it wasn't running as "crisp" as it should. Today, I did some investigating and this is what my spark plugs look like (NGK Irridiums):
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To my untrained eye, Cylinder 3 is obviously sick, and the others seem to be running lean (am I correct to assume this?). Among other things, during the rebuild, here is a list of some of the related things that were replaced and done:

• New Deves Piston Rings, cylinders 1 & 3 liners replaced..rest were just honed to deglaze, new big-end rod bearings
• Heads rebuilt from local reputable machine shiop, new guides, oil seals, & 4 bent valves were replaced (cyl 1 & 3)
• 8 New Bosch Fuel Injectors
• New Ignition wires, inspected extenders, no cracks under magnification...decided to reuse
• All new gaskets, seals, belts, hoses, filter, and everything that "should" have been done to be on the safe side.

A quick compression test of all cylinders yeilded the following results (cyl-pressure):
1-152 2-148 3-151 4-149 5-150 6-149 7-149 8-148

I am assuming that cyl 1 and 3 have markededly higher numbers becauser they received new cylinder liners and the rings have seated quicker than in the ones I carefully honed (deglazed), but there seems to be plenty of compression in the cylinder that is producting a black plug. What I don't understand, is how this one cylinder might be looking like ti is really rich. Can this happen to a plug if it is not firing all the time due to a bad extender, plug wire, or something like this?

I'm no pro, yet this is my wild theory, and I welcome any input. I'm thinking that intermittant spark is happening in this cylinder, and the unburned air/fuel is causing the lambda system to detect a "too rich" condition and lean out the mixture.....and is why the other plugs to look as they do. I am probably way off with that, and welcome any ideas as to why this plug is black, and where to go next in my troubleshooting? Thanks in advance.

 

I totally understand that the first suspect would be the injectors....but the thing that is stumping me is that they ARE new. Before I installed them, I did to a spray pattern check & made sure they were all putting out the same volume (all injectors in a cup while pressing down on distributor plate). Could this be due to a bad fuel distributor? Are each of the 8 fuel output ports (that connect to the injectors) independantly metered, or is is a common "bus" which is like an 8-way tee. Is is possible for one outlet on the fuel distributor not supply fuel, or supply a volume or pressure which is different than the others?

 

Hmmm....Interesting. Are you pointing at an intake or exhaust manifold? I did notice a slight "ticking" sound coming from the rear bank recently, and it is not coming from the valves (I tried locating it with a mechanic's stethascope). I have seen ex manifold leaks make a ticking sound before, but could a small leak cause cyl 3 to run that rich (black)? I do have all new gaskets, including exhaust, but I know that this does not excuse me from having a leak.

When the car is warming up, during fast idle, is when I notice that the engine is "missing" the most. Instead of a smooth note, every second or two, the tone drops lower for a fraction of a second which sounds like a misfire. I guess I'm going to re-check injector flow & spray patterns, and I just bit the bullet & ordered some new extenders. My dist caps and rotors are new as well.

Thanks guys.

 

Yep, most of you were right on the money by saying this was ignition related. I was scratching my head like crazy becasue I changed out the cyl 3 extender, checked the resistance of the wires (no probs found), and cyl 3 still wasn't firing all the time. When I checked the resistance again, I got the correct 950ohms, for these wires, but when I jiggled the leads, it jumped to 2.7K. In case anyone lands on this thread chasing down a misfire, I just wanted to say that I found out the hard way that even though your wires might ohm-out correctly, make sure you have SOLID connections. I made up a new wire for #3 and she is running like a top now! Thanks all.

On a side note, I know this sounds like a stuipid thing but....I have also found that if you are not REALLY careful, it is possible to completely "miss" getting the spark plug wire on the extender when you are using the large rubber caps. There is plenty of room for the externder tip to "flop over" and messle into the side-space of the cap, and it will feel like it is on correctly. I did this a couple times, but quickly caqght it (and am now admiting it!).