Spark Plug Reading - Gas Engines - 4 Stroke
Example1:
#1 Is a timing indicator, you'll see a definite color change on the ground strap, it doesn't show well here but you can still see it right about at the arrow. Too much timing and the color change will be very close to the threaded body of the plug, too little and it'll be closer to the tip. Ideally we want it right in the apex or center of the 90 bend on the ground strap. This plug shows too much timing for the combustion chamber efficiency or octane level.
#4 Arrow shows another indicator of timing, you'll usually see a brown ring right at the tip of the porcelain area it should be a sharp and defined ring about .020 wide. Wider indicates not enough timing and any smaller , or only 1/2 way around or nonexistent as in this image is the second indication of too much timing in the motor.
#2 The tip of the ground strap is loaded with OIL deposits, fuel deposits are usually flat black in color and almost like a fine powdery deposit, this motor is leaking oil into the combustion chamber, bad valve guides, leaking valve covers allowing oil to seep through the plug threads, whatever it needs to be fixed.
#3 The threaded portion of the plug gives you the heat range, look at the threads you'll see that a few toward the tip are a dull burnt looking color the rest are black and shiny. You want about 2 threads showing the heat on the end of the plug and the rest of the threads to be shiny, this plug is impossible to read because of the oil mess. If you using a longer reach plug than this one 2.5 to 3 threads is optimum.
To increase the number of burnt threads increase the heat range of the plug, if you have 4-5-6 threads burnt you need to get a colder plug.
Looking at the color of the porcelain I'd give this carb a passing grade at the mid range and not to bad on the idle circuits although that dam oil leak makes it tough to really get a good read. I'll get some better shots for lesson 2.
I'm going to keep working on this page and try and get some real good shots of various plugs, we'll study each one and find the good and bad tell tales of each.
I'll also start pulling some plugs out of my race car at various settings and get pictures so we can see how jetting, timing and heat range affect the plugs in the same engine under the same conditions.
Example 2:
that it's slightly fat at idle.
The white porcelain is showing a lean condition at WOT, it's not too far advanced as the total timing mark or color change is right in the apex of the ground strap curve. The Idle timing is shown by the triangular hazing up on the flat of the ground strap and without even looking at the distributor specs I can tell you that the timing on this SB Mopar is about 18-20 initial and 34 total.
I would need a better picture of the threads to determine the heat range.
Changes:
I would try and lean it out just a touch at idle and up the jets by 2 points to fatten up the WOT circuit.
That slightly lighter color at the tip of the ground strap indicates too much gap, nothing serious but next time you change plugs I'd go to about a .036 gap from the current .040. Too much resistance caused by too wide of a plug gap can cause excessive heat on the tip which will shorten the life of the plug and really give you no benefits. I believe excessive plug gaps are not required on most Muscle and bracket cars, once you get into real big compression and major power you would open up the gap and replace plugs 2-3-4 times a year.
Example 3:
Note the change of color on the ground strap and it's position, this shows a timing issue. Unfortunately the reader who sent this image in didn't say what engine he was working on but by the condition of the plug I would guess a Small Block Chevy, 3 years old with 38* total timing and an Edelbrock Carb......I'm sure he will respond :-)
In the second image I used a green arrow to show the area where we want the timing mark to be on this engine and of course you can see that the motor has too much timing.
Image 3 shows only one thread heated on the plug and rest are oil soaked and wet this normally indicates that the plug is too cold, the oil indicates a valve cover leak and a New set of Cometic Aramid fiber gaskets would cure it.
The color on the plug indicates a decent idle A/F ratio and a good ignition system probably an MSD or some other type of aftermarket ignition. The hot spark is keeping the porcelain clean on the top area anyhow. Now if we look down into the bottom of the plug as seen in the above image where the porcelain reaches below the steel body we see black deposits indicating a rich condition at mid range to WOT. I would guess this engine is a little lazy in mid range and could get better mileage.
Changes:
Back off the total timing but keep whatever the initial is as it appears pretty good, I would shorten the total by 4* and do another check.
Fix the oil leak
Go up one heat range and after about 50 miles check to see if you have 2-3 threads heated up and discolored for correct heat range. Heating up the plug will help reduce the carbon build up and further tuning with the A/F ratios can be done
Before you can really tell what to do with the carb the ignition must be right as well as the correct heat range on the plug to get true readings.
...and check your Idler arm it appears to have some slack in it and your left rear tire needs 3# of air.
Plug Characteristics
Shiny or Glazed Porcelain: This occurs for two reasons, excessive idling or rich idle condition leaving heavy soot deposits. These deposits will burn and create excessive porcelain temps causing the glass to melt usually resulting in a down track popping through the exhaust..
You will usually see splotchy deposits on the plugs after the run, either green or yellow in color and of course the tell tale shiny glaze. This glazing of the porcelain actually becomes a conductor and the spark will jump from the electrode to the porcelain resulting is a very weak small if any spark. The porcelain should always have a chalky or dull finish.
Base of threads sooty: This is usually a indication of the plug not being tight enough allowing it allow air to pulled in to the chamber and fuel being pushed out, change the plug, tighten to spec. This area should have a nice dark to medium gray color without heavy deposits.
Black or brown specs: This is an indication of detonation usually caused by too hot of a plug, drop one or two heat ranges and it should go away. The plug gets so hot that it will start to detonate the fuel before the ignition fires, this cause a double flame front and reduces the efficiency of the combustion process as these two flame fronts battle each other in the combustion chamber. The black spots are the result of the fuel deposits being burned onto the porcelain by the double flame front. Once you cool down the plug and remove the secondary ignition source you can re-set your timing to produce the correct combustion chamber temp for optimum power.
I've heard 100's of so called tuners and engine builders tell people that this is normal and nothing to worry about......WRONG. Do not underestimate the damage that this condition can do to your engine, drop the plug heat range.
Shiny little diamond like deposits: It's too late you've already started to blow the pistons out of it. Start over from scratch, fatten the carb, back-up the timing and cool down the plugs. Now start a proper tuning procedure and hopefully you caught it before too much damage was done.
No color mark on ground strap: If your showing good color on the porcelain but the ground strap fails to show an indicator then your plug is too cold, go up 1 heat range.
If the ground strap is showing a good color line but the porcelain is showing no color at the base then your plug may be too hot, drop down a range.
I will use maybe 3 different ranges on one bank, make a pass and from those three different plugs determine which one is correct for the engine.
Variable heat readings: On a small block motor most have adjoining exhaust ports, these will always be your hottest cylinders and in some cases where there has been excessive port work and thinning of the metal between the ports it may be necessary to run a plug one step cooler in those cylinders, use a plug indexing washer to back the timing down in those cylinders slightly, or if you have an adjustable ignition system you may want to electronically back the timing down in the hot cylinders.
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