Spark Plugs Too Dark... Ideas?

[sopwith21]

Howdy all;

I have an engine from a 79-83 280zx, used for racing. Here are some basics on the build:

o Bore: 86.75 mm (+.030”)

o Stroke: 79 mm (stock)

o Compression: 11.9:1, or so the builder tells me

o Camshaft: .560” lift at valve, 300 degrees duration

o Valves: standard

o Bearings: standard

o Cylinder head is an “N42” from 1975-80 280zx

The engine only has about 100 race miles on it but the plugs are fouled. They appear dark with and covered in soot. The plugs are NGK BP6E S.

The motor runs very well, but I'm clearly not getting peak performance from it. I've checked the ignition cap and it has no cracks. The rotor is not fouled and is firing cleanly.

All six plugs appear to be fouling at roughly the same rate, so I don't think it's a cylinder issue. The air filters are as old as dirt, but they seem to be flowing well and not overly restricted.

The motor is so smooth at hi revs and at idle that a carb issue seems unlikely, but ??? I'm running a typical two-carb Datsun setup.

Could these plugs be too hot? Any advice is appreciated. Thanks!

[SteveJ]

You might try colortune to see how rich you're running. You could also try welding a bung into your exhaust for an oxygen sensor.

Where did you source the carburetors from? And by "a typical two-carb Datsun setup" are you referring to SU style carburetors?

[sopwith21]

Yes, twin SU carbs. Don't know beans about tuning them. I can barely get a Holley 600 to run right!

[SteveJ]

Then track down user Bruce Palmer here. Tell him you want the ZTherapy video on tuning SU carburetors.

[Travel'n Man]

Yes, twin SU carbs. Don't know beans about tuning them. I can barely get a Holley 600 to run right!

Get the video and a Air/Fuel gauge - dial those baby's in - it's that easy.

[bartsscooterservice]

Do you have a picture of the plugs? It's actually okay to have a little but to much fuel but not to much, a dark grey to dry light dark is what your looking for. Heat range 6 should be fine, ngk recommends this as stock.

[sopwith21]

No pic, but they are not dark grey. They're black and I can wipe a small amount of residue off with my finger.

[SteveJ]

Oily? Are you sure your oil rings are offset enough?

[bartsscooterservice]

Seems like it needs a smaller main jet then. I do think if stock is heat range 6, and your slamming it on the track high rpm, heat range 7 would be better. I'm not an expert in this, but would'n't a stronger amp ignition be better for a more complete burn ?, it can make a difference in spark color output..

Edited February 12, 2013 by bartsscooterservice