L26 Misfire & A Plug Reading Question

[Murph]

Looking for a little help to narrow this one down before I start taking pot shots.

 

A little background. Stock L26, 240Z hitachis, electronic ignition, starting to get a little tired, but still runs strong.

About 6 months ago, out of no where the car developed a big misfire. All revs, all loads, plugs on the front three cylinders quite white (rear three I recall were normal tan colour). Wound in $^!# load more fuel on the front carb, no difference in plug colour, still missing. I was too busy to deal with it, so parked the car up.

 

3 weeks ago, pulled the car out, took it for a drive and it seemed ok. Just a little bit of a miss up in the revs sometimes. Gave it a quick tune and I've driving it like this for three weeks now.

 

Tonight, the car was running nicely. Gave it a hit up through 2nd gear and it pulled nicely. Turned into the left hand corner at the end of this straight and it started to miss again, just like the issue from six months earlier. Any revs, any load from idle to WOT and it'll miss. Intermittent pattern, say 1-3 misses per second. Enough to make it a PITA to drive, but not so bad that I couldn't get home. Giving it a squeeze in third it still pulled hard, just with an intermittent stumble.

 

Got home, pulled the plugs and they're all white again. This time it's all of them. The front three are just little whiter than the rear three, but not by much. I know that I had both banks tuned on the rich side of things, with a fairly dark tan colour.

 

So, what do white plugs tell me? As far as I know, that's either lean, or over-advanced ignition. Lean just doesn't make sense. The only fuel related issue that could affect both carbs at the same time is fuel starvation, and that doesn't make sense when the miss happens at idle as well as at WOT (while still making decent power).

 

So if it's ignition, a weak spark would leave you with darker coloured plugs wouldn't it, not lighter?

Am I right to go chasing a timing related issue?

 

[Zed Head]

Before you chase you might just check the basic ignition parts, like the cap, the rotor, grounds, module connections, etc.  What type of electronic ignition is it?

 

Consider bad gas or a clogged fuel filter also.  A solid overall tuneup before getting carried away might be a good idea.  I remember once spending hours working on a car before realizing it might just need new spark plugs,  Which turned out to be the problem.

 

In your case, those plugs are pretty dirty.  The carbon is a short circuit for spark.  Once they foul, they're hard to get clean.

 

I'd do a tuneup first.

 

 

 

 

[Murph]

Good advice there mate. Off to limp it down to the parts shop to start with the basics.

[Murph]

Plugs, rotor & cap....sorted. Just the way the miss came and went doesn't quite make sense to me.

[Zed Head]

Sorted means fixed?  Who knows, when the problem is on the edge of being full time, maybe simple stuff like humidity plays a part.  Spark doesn't care how it gets to ground, it's just gotta get there.

 

 

Another memory from my super-economical past (high school) - I used to carefully lay out the old crusty spark plug wires on my chevy engine to keep the misfires away.  Plug wires were too much money.

Edited April 1, 2015 by Zed Head