Plug colours

[Roger]

I have recently fitted a SU 1 1/4” carb which came with a 
BP needle fitted it gives a marked improvement to performance and starting.
I have also fitted a manual override to the distributor, this is a great help to find optimum timing setting.
All appears well except No. 3 plug always is very rich in appearance the other three look reasonably ok. The photo does not show how sooty No. 3 is compared to the others.
Does anyone have  possible reasons that may cause this? I have skimmed the block face and manifold to avoid leaks, the carb has a new home made adapter all faces are flat.
All the plugs are the same Champion D16 with 0.020” gap.

Roger

   

[steve anderson]

OIL  ???

[Bob Culver]

The compression on the handle may be a clue to a broken ring, but a broken oil ring not detectable.

[John Cornforth]

Oil tends to give a shinier black look, its hard to tell from a photo. You seem to have what's described as dry carbon deposits. This could be over-rich mixture, but one would expect that to affect all of the plugs. An occasional misfire on that cylinder is another possibility, so check distributor cap, plug lead and plug gap.

It's sometimes the case that plugs soot up from rich mixture at idle. Ideally this gets burnt off by sustained running to get the plug tips hot enough, not always easy with a Seven. Some advise a plug reading technique which involves cutting the ignition when running and coasting to a stop to avoid an idling phase, but you need a quiet road !

[Roger]

John,
I tried your cutting ignition at high speed after a good distance so engine quite hot, although not running particularly hot in general.
Plug three always appears a very dry matte black, I dismissed oil as I would expect that to be somewhat wetter looking.
I have swapped plugs around but it is always No. 3 that is like this.
Oil consumption is  very little, about 1/8" down on dip stick after about 500 miles since rebuild, rebore, new pistons rings (correct way up) etc..
Cylinder pressures are all about 100psi.
The mixture may be rich, so maybe the question is, why are the others showing weak mixture.
I have checked all joints so no air leaks.
I believe I had been running slightly too advanced, I fitted a manual control so I could check this and was able to find a good position so the engine runs quieter now, but not too retarded.
The Ignition is something I am checking but there does not appear to be any misfiring, performance is good, although I am not an experienced A7 owner. I do not recall this happening when I had the Zenith Carb fitted but had less reason to check then.

Thank you for the replies.
Roger

[MartinH]

Weak(er) spark on no.3? Swop plugs around. Check HT leads and dizzy cap.

[dickie65]

Hi
I have just had a look at the Minty Lamb SU chart and your needle seems to be on the rich side for an untuned engine ?
What spec is the engine ??
I am using an E2 needle which is slightly weaker at the top end on a tuned pressure fed engine capable of 6000 RPM
I have a cut down spring fitted.
Are all your valve guides in good condition ?

[Roger]

Martin,
I have swapped plugs around, still the same results.
I will try new Distributor Cap and lead tomorrow, weather permitting.

Dickie,
Valves, seats and guides all new. engine standard (+60 bore).
Yes my needle is possibly rich, so what is happening to 1,2 and 3 plugs. .
If they were all rich my choice would just be swap needle, easy.
I have fitted a new jet which is centered ok.

Roger

[Howard Wright]

Hi Roger

Interested in this thread as I have a similar problem.

It has been suggested to me that a sticking valve could cause plug sooting.

My engine is running well so at the moment I’m leaving things alone.

Cheers

Howard

[Roger]

Good morning Howard,

I did wonder about valves. A while ago the engine was a bit sluggish so I checked the cylinder pressures and found one very low. The tappet nut was loose and caused the valve not to lift, so no air going in the cylinder to be compressed, hence the low reading.
A similar situation would make No. 3 cylinder short of air giving a false richness!
I took the manifold off to check for leaks and faced it off by a few thou on Friday, why didn’t I check the valves then
Today’s job = check cylinder pressures.

Roger