half black insulator on anode of spark plug?

Stock FSJ Tech Area
Post Reply

Topic author
marc
Posts: 178
Joined: Tue Aug 11, 2020 8:24 am

half black insulator on anode of spark plug?

Post by marc »

So far I have pulled the plugs on four of the six cylinders of my j10-258 and all show blackening on one side. (see photo) Dry compression on all four of these cylinders is between 120-130. I gather that asymetric blackening of the plug can be a symptom of burned exhaust valves but the compression seems ok in all so far tested. Can anything else cause this phenomena? For a while the distributor cap was loose on one side, the truck would stall immeditely after going over one particular speed bump. With the cap firmly screwed down, it no longer does this. Currently running Autolite AP985 plugs. Teh truck now starts at the first try and no longer surges when cold. It does shudder to a stall when cold once of twice after startup. Thanks for any thoughts on this.
Marc
spark plug.jpg
You do not have the required permissions to view the files attached to this post.
J10 1984, very stock with AMC 258.
Truck appears to have been rebuilt or restored at one time
User avatar

Stuka
Site Admin
Posts: 11789
Joined: Thu May 12, 2011 5:53 pm
Location: CA
Contact:

Re: half black insulator on anode of spark plug?

Post by Stuka »

That typically means you idle is rich. The other side being white may indicate that you are lean at higher throttle.
2017 JKU Rubicon
Pevious Jeeps: 1981 J10, 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ

Topic author
marc
Posts: 178
Joined: Tue Aug 11, 2020 8:24 am

Re: half black insulator on anode of spark plug?

Post by marc »

I am wondering if the plugs might be too cold. Had been running NGKs and they showed less of this 1/2&1/2 color on the anode. If the entire plug was as white as the white half of these Autolites, it would clearly be burning too cold. Naggng thought, could the valves be slightly out of adjustment and cause this? I am going to change out the distributor rotor and cap just in case bouncing around with one side loose has compromised them.
J10 1984, very stock with AMC 258.
Truck appears to have been rebuilt or restored at one time

letank
Posts: 4009
Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

Re: half black insulator on anode of spark plug?

Post by letank »

Post a pict of the frontal side of the anode, it can show a lot more, such as timing, perfect timing as you can see the color change around the arc of the anode here for my motorcycle, a tad rich at idle
XS#12021Spark.jpg
You do not have the required permissions to view the files attached to this post.
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (229 Kmiles... the running test lab)

Topic author
marc
Posts: 178
Joined: Tue Aug 11, 2020 8:24 am

Re: half black insulator on anode of spark plug?

Post by marc »

Not sure what the "frontal side of the anode" is, the gap side? Here are two pics of one of the NGK plugs I replaced with autolites.
plug 3.jpg
plug 1.jpg
You do not have the required permissions to view the files attached to this post.
J10 1984, very stock with AMC 258.
Truck appears to have been rebuilt or restored at one time

letank
Posts: 4009
Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

Re: half black insulator on anode of spark plug?

Post by letank »

Looks good for timing.

Here is the link for spark plug reading:

https://fboignition.com/articles/sparkplugreading.html
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (229 Kmiles... the running test lab)

letank
Posts: 4009
Joined: Wed Oct 03, 2012 9:16 pm
Location: SF bay area

Re: half black insulator on anode of spark plug?

Post by letank »

found my 85 plug, champion RN12YC, with over 5000 miles
perfectSparkBroFSJs.jpg
You do not have the required permissions to view the files attached to this post.
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (229 Kmiles... the running test lab)

Topic author
marc
Posts: 178
Joined: Tue Aug 11, 2020 8:24 am

Re: half black insulator on anode of spark plug?

Post by marc »

A follow up that might be interesting. The old distributor cap, having stripped threads on the shallow hold down screws, was loose on one side. Should have noticed that the front screw had already been replaced with a longer than stock one. Replaced the cap and rotor and the truck is running noticeably better, smoother and not coming to a shuddering stall once or twice as it warms up. As I mentioned, the most obvious sign of the distributor cap issue was the vehicle stalling immediately after going over one particular speed bump. Replaced the cap because I could not imagine jolting over rough pavement with a loose cap not causing some kind of damage.
Thanks for the reference to plug reading, i will study it.
Marc
J10 1984, very stock with AMC 258.
Truck appears to have been rebuilt or restored at one time
User avatar

Stuka
Site Admin
Posts: 11789
Joined: Thu May 12, 2011 5:53 pm
Location: CA
Contact:

Re: half black insulator on anode of spark plug?

Post by Stuka »

Ahh, that would do it. Glad you figured it out.
2017 JKU Rubicon
Pevious Jeeps: 1981 J10, 1975 Cherokee, 2008 JK, 2005 KJ, 1989 XJ
Post Reply