Reading Spark Plug

[Helix]

After the exhaust and jet kit. I wanna see where my motor stands on the a/f ratio. I installed a new plug ran it a WOT pulled in the clutch and hit the kill switch. I looked at the plug but hard to tell. Theres not much color on the plug. Looks lean to me. But the plug is brand new. Do I need to wait and run the plug for a little longer and wait for the plug to color up to see what the motors doing? But if its lean I don't want to run it to much. I have a 170 main in right now. HMF full system, k&n with outterwear. 2300 ft. above sea level. Sucks cause the 170 jet is the biggest that came with my dynojet kit. If its lean I need to buy some bigger jets.

[Rip_Tear]

It didn't take long for my plug to change colours... How did the machine run?

[GPracer2500]

Sounds like your doing it correctly (new plug with zero run time except for the WOT test run--warm up the engine on a different plug) but your missing one thing. You've got to look way down inside the plug at the base of the insulator to see what you need to see. Many a tuner have gotten the "must use a brand new plug" part of it right, but mistakenly kept going richer and richer because they weren't looking at the right part of the plug. A performance enthusiast reading a plug to determine if he needs a 165 or 170 main jet is not the same thing as reading a plug to figure out why your tractor runs like junk. That's not a jab at you, but rather a jab at the "common knowledge" we've all heard about reading spark plugs. Forget those useless charts that only show plug tips. Those are for commercial mechanics diagnosing engine problems, not performance enthusiasts trying to fine-tune a carburetor.

They call it a "plug chop" because in many cases you have to actually chop the threads off the plug to see what you need to see. Sometimes a magnifying glass and a pen light are enough to see down there without cutting the plug.

Here's some examples of plugs that have been chopped to view the mixture ring at the base of the insulator:

http://img62.imageshack.us/img62/4870/plug148mainmediumrh3.jpg

http://img62.imageshack.us/img62/1542/plug152mainmediumoi2.jpg

http://www.dfn.com/benkaren/Plugs.JPG

http://www.dragstuff.com/images/plugs/10-29-05pass1-05.jpg

http://www.dragstuff.com/techarticles/images/sparkplugs/AR3933-terry-wise-plug2.jpg


It doesn't take long at all to get the plug to "show" a mixture ring at the bottom. The first two plugs where each colored with a trio of 7 second WOT passes, one after the other in rapid succession. Those plugs show main jets 2 sizes apart. The first looks good. The second looks rich.

Different bikes and different fuels may react a slightly differently, but it won't take long at all for the mixture ring to appear. Running a plug for long enough to get it to show color all the way to the tip of the plug doesn't give you detailed enough information to make anything but generalized decisions on jetting. By the time the whole insulator is colored you'd have very likely run the engine at more than one throttle position, load, engine temp, etc. All that gives you is a confused "average" that is hard to base accurate decisions on.

[GOTFEAR]

Nice throw the magnifying glass away and flash light

[Helix]

Can you suggest the best way to remove the threads?

[GPracer2500]

Originally posted by Helix
Can you suggest the best way to remove the threads?

A hacksaw will work. A Dremel with a cut-off wheel is faster. Really, any tool that will cut metal should work. Cut into the threads anywhere from the first thread up to maybe 5 or 6 threads up (depends on the plug). Cut all the way around and through the metal until you reach the ceramic insulator embedded within. Then you can pull the threads off, exposing the ceramic insulator underneath.

[GPracer2500]

This plug had many hours of runtime on it when it was cut. So, it is not particularly reliable for carb circuit tuning. But I'm posting it to further illustrate where you should be looking. I cut this one with a Dremel. Somewhere in the area between the red arrows is where the "mixture ring" will be located. Different engines, different plugs, different operating conditions, different test procedures, different fuels, etc. etc. can all add some variance to precisely where and what you should be looking at and what it means. In many cases it makes sense to get a bunch of plugs and do one big initial effort to figure out exactly what test procedure will produce good, readable indicators on YOUR plugs in YOUR engine in YOUR conditions. Once you've got that experience future readings will go faster and easier without wasting plugs or time.

http://i34.photobucket.com/albums/d133/gpracer2500/plugcutMedium.jpg





BTW, in case anyone is wondering: the greenish/yellowish/browish area towards the tip of the insulator are lead salt deposits from running a leaded fuel. This plug was banging around in a tool box for a while. Right after it was pulled out of the engine those deposits where a much brighter green/yellow color. This is sometimes called lead glazing. Leaded fuels have chemical scavengers in them to help evacuate lead deposits with the exhaust gases, but over time they can still build up (usually no more so than any other kind of deposit--in other words it's not reason to shy away from leaded fuels).

The first time I saw green deposits on a plug I was like--WTF!?!? :p