Spark Plug Colour

[William2]

Having got my engine to run smoothly without any faltering on acceleration, I took the plugs out after a decent drive. All four plugs had electrodes that were brownish/grey in colour and the outer plug rims were black. Does that mean the mixture is probably about right or a bit rich?

[Grizzly]

Ideally their supposed to be a Tan/Sand colour after a run but whats more important is that their all the same colour and the car runs properly. Assuming your on Webers to change the running mixture requires a jet change so if you haven't messed with them and their all clean/Pump jets are clean and working you should be fine.

You can of course get it rolling road set up to optimise the jets etc but if it's balanced and Brown not black i'd leave it (better to be a touch rich than too lean)

[Galwaylotus]

Having got my engine to run smoothly without any faltering on acceleration, I took the plugs out after a decent drive. All four plugs had electrodes that were brownish/grey in colour and the outer plug rims were black. Does that mean the mixture is probably about right or a bit rich?

The correct surface to examine is the ceramic insulator surrounding the centre electrode. It should be a tan to chocolate brown colour.

[bill308]

Hi William2.

I think on balance, you are about right. To do better, I would need real data.

I would use a Colortune for idle mixture and set a mixture right at the blue/yellow boundary for starters, then work from this reference.

For off idle, I would use an O2 meter, like an LM-1 or LM-2 or something similar, measuring exhaust gas from a bung in the header collector. You just can't beat real data.

Alternatively, an extended session on a rolling road with an experienced tuner might also be worthwhile.

Bill