Itinerant Air-Cooled

While examining my spark plugs, I found something curious.

The electrodes on all four plugs appear the same - a flat, light gray color. The center insulators are all still basically clean. (The plugs have only a few thousand miles on them.)

Cylinders 1, 2, and 4 have a very minimal light coating of carbon on the bottom of the threaded base of the plug.

But, #3 has a much heavier dark fluffy coat of carbon on the bottom of the base.

Would this indicate the #3 is running richer than the other cylinders for some reason?

I had my fuel injectors cleaned and tested by Cruizin' Performance a couple of years ago and they all were very equally matched.

I'm considering swapping the injectors for #3 and #4 to see if the additional carbon buildup moves with the injector.

Any other suggestions? I don't see how the air intake to #3 could be restricted.

Hows compression and/or 'spark' intensity?

Do you have a CHT?

I like the swap method but would check these things first.

Doesn't really matter whats on the end of the threads. What matters is the condition of the electrodes and the insulator. The bit of fluff may be due to different depth of spark plug holes aloowing a wee bit more of the threads being exposed in th chamber, or the plug gap is facing a direction that is not optimum for ignition. Google indexing spark plugs.

JMHO not necessarily gospel.

Very interesting! I had never heard of plug indexing before. I'll have to give that a try!

For the moment, I'm just driving and enjoying it!

You might check the resistance of each plug end. A higher resistance value will cause it also. Look on the plug end edge. It will have the value. More than likely, it is 1.4K ohm. [1400]

As Gypsie asked about CHT, I'm wondering if you have the CHT sensor on #3 and if that could be a factor.

Ah, sorry, I forgot I had not responded about that. I do need to run a compression check, and I do not have a CHT gauge or sensor - and don't want one!!!

vwlover77 wrote:Ah, sorry, I forgot I had not responded about that. I do need to run a compression check, and I do not have a CHT gauge or sensor - and don't want one!!!

If your engine is running smoothly with a steady idle and decent pick up, the chances of different mixture in one cylinder is much less likely than a set of rings giving you more oil (we're not talking "wet plug" just a little vapor) that promotes carbon build-up. As noted, the color of the electrode ceramic is the key determination of mixture. The periphery of the plug gives you more of a snapshot of conditions in the cylinder, compression and oil rings, cylinder cooling, exhaust manifold leaks, etc.
Colin