If I'm using a spark tester with an adjustable gap set to the correct gap for my car's ignition system voltage, what should the spark look like?

I ask because I've seen conflicting opinions on this matter. For example, this Brigg's and Stratton site says:

A bright blue spark is best. A yellow/orange spark signifies weak ignition. Not true. Spark color determines virtually nothing. The hottest spark is ultraviolet which we can't see. Blue spark is cold in comparison to ultra-violet. Orange and yellow come from particles of sodium in the air ionizing in the high energy of the spark gap.

However, they are talking about small air cooled engines, and are using a specific Briggs and Stratton spark tester with a fixed gap for small air cooled engines, the 19368 model.

Here's another person saying spark color doesn't matter:

I might note though that the very fast, very high voltage sparks in todays systems usually are not blue. Your mileage may vary.

This poster on another site responding to a similar question about the above Briggs and Stratton quote says:

On an automobile ignition system the spark should be bright blue. That's because the compression ratio is higher than that on an air cooled small engine. An automobile engine operates under a transient condition where the RMP's change so fuel demand changes. Where with most air cooled small engines they operate under a static condition or a steady state.

Compression ratio of an engine and the amount of fuel being delivered can have an effect on how well the spark plug fires. This is called quenching the spark. A low compression engine running at a steady state with a constant fuel supply won't quench even a spark plug with a yellow spark. But on an automotive engine with a higher compresssion ratio and under transient conditions the yellow spark can get quenched thereby causing a misfire.

Here's another guy saying basically the same thing:

The recommended 7/16" gap has always worked for me on outboard motors. I use mine in line between the spark plug installed normally in the engine and the plug wire. This means that the spark must jump the 7/16" gap and the spark plug gap in the motor while running. You can then also increase engine rpm to verify spark at higher speeds. My experience has shown a strong blue spark with a noticeable popping noise in a healthy ignition. The gap is determined by the amount of voltage generated by the ignition, spark plug gap, and the compression ratio. Most manuals will give an air gap recommendation for a spark test. An air/gas mixture at high pressure creates a much denser gap for the spark to jump then in out in the open.

I don't have the background to tell who's right and was wondering if someone more knowledgeable could give me some verification here.

As a side note 7/16th is about 1.1cm, and the breakdown voltage for air at sea level is about 30kv/cm. Also of note is this answer on the physics stack exchange which says that the blue color of air sparks comes from ionization of nitrogen atoms.

One other point of interest is that the power of light is inversely proportional to it's wavelength, meaning blue light is about 40% more powerful than orange light based on the ratio of their wavelengths.

In fact, someone else has basically asked a very similar question over on the physics stack exchange:

Is their a visual difference between air-gap sparks of the same voltage but different current?

Has anyone had personal experience with seeing a yellow/orange spark, doing a repair and then seeing a nice strong blue spark with a disappearance of the performance problem?

  • I realize the above is probably a cut-n-paste, but is that supposed to be "RPM's" and not "RMP'S"? Mar 19 '16 at 20:15
  • +1 Nice question, hopefully respondents will provide citations. Mar 19 '16 at 20:30
  • @Pᴀᴜʟsᴛᴇʀ2 Probably, but as you said, it's cut and paste. Mar 19 '16 at 20:38
  • The spark you will see in a spark tester won't be under the same conditions as inside the cylinder, so you wouldn't expect to see the same thing. Do you think?
    – HandyHowie
    Mar 19 '16 at 22:34
  • 1
    When I was about 14, which was 62 years ago, I hung out some at the local garage. That was back in the days when spark plugs became bad they were cleaned in a spark plug sand blaster. Money was a little tighter back then. After cleaning, Ethimer would screw them into a pressurized container with a window to check if they fired. Explained that under pressure a border line plug would misfire. I cleaned and checked quite a few plugs. A lot has been lost in 62 years sometimes.
    – trailscout
    Aug 28 '18 at 23:18

Many of years ago I had a moped which would not start. The helpful agent asked me "have you got a spark?" "Yes" I replied.. What colour is it?" he asked. "Blue with yellow streaks" I said. .. "Sounds like you need new ignition coil, as the old one is breaking down internally .. burnt out" he said. Consequently I changed coil, and engine burst into life again. I later stripped the old coil, and sure enough coil internal wire insulation was roasted and shorting.... Bob .. retired armature winder.

  • What color was the spark after swapping the engine coil?
    – Ferrybig
    Mar 16 '19 at 13:55

Spark testers are used (on cars) to indicate presence of a spark only; there's no other diagnostics you can do at that point. As long as the gap is set correctly (err on the tight side if you have to - it will expand/erode over time), and you have a spark, you're good to go. If the car still won't start, it's something else; fuel and/or air.

Small engines such as Briggs & Stratton might have a preference for color, but they're a different beast.

  • The whole point of this question is that there is a dispute about the claim that you are making in your answer and I am looking for concrete evidence one way or the other. Mar 23 '16 at 7:50

A good spark plug tester will pressurize it's test cylinder to the same pressure as engine combustion chamber. For a given spark gap test an yellow spark against a blue spark. As the gap or pressure is increased the snapping blue spark will win.


Automobile ignition generates more voltage than a magnito ignition, since automobiles have multiples of cylinders and have more production in power and carrying loads so they have ignition timing that adjusts to the various loads and speeds but on a lawnmower it really don't matter what color the spark is so it runs, but however on a automobile a strong healthy spark is blue and a weak spark would be an orange but will still work but not effective. Orange sparks are like slow sparks that miss fire and creates other problems, so blue is more effective and that's how ignitions are designed to properly be efficient. Orange sparks could mean ignition problem in the coil, a bad ground or too much resistance.

  • The whole point of this question is that there is a dispute about the claim that you are making in your answer and I am looking for concrete evidence one way or the other. Thanks for taking interest in the question and participating. Jan 15 '17 at 5:17

There is a system that has been available for years that allows you to see the colour of the spark inside the combustion chamber. The Colortune.

This is specifically intended to assist in tuning the engine and, yes, it does help cure tuning problems.

I haven't used one for years but they were de rigeur in the days before engine management systems.

  • 2
    The Colortune isn't to determine the color of the spark, but for the color of the combustion itself, which is useful for tuning fuel/timing (leaner mixtures will be bluer in color, where richer mixtures are redder.
    – Shamtam
    Mar 23 '16 at 20:12

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