Spark color to determine ingition coil performance

Good Morning;

I am having some minor power issues with an older vehicle. I found that some of the wires looked bad and replaced them. I replaced the plugs, cap and rotor about two months ago. The alterantor and battery within the last year. It seems to be running better but I am still looking at it this Christmas weekend.

In researching the electrical system I came across this link by briggs and stratton.

Towards the bottom of the page is a myth section which includes information on spark color. I was considering replacing the coil this morning until I had read this. About 20 minutes after dawn the spark color came through orange (exactly orange) in the shade with poor grounding to a painted surface. At the moment I am alone working on the vehicle so I put an old (good) plug in the coil lead and placed a pair of channel locks on them against the grill near the windshield so I could see the spark as I turned the engine.

Can anyone verify this information. I have always had the understanding that the spark color should be as bright a blue as possible (to non existent except at night) in understanding that the coil is performing well or better.

I’m reluctant to spend Lord knows how much until I better understand this. Though I suppose paying the Lord knows now will save me 10 times as much for a tow these days :slight_smile:

Thanks for your help. Have a Great Day!!

Frank Pytel

A spark is a spark is a spark, and it either ignites the fuel or it doesn’t. Once the fuel is ignited, the spark does not matter any more, it has done its job.

I’m sure you have heard the term “pushing the envelope”. With ignition, there is an envelope. With an optimum air/fuel ratio, there is a wide range of spark that will ignite the mixture. I don’t think there is too much spark, but there certainly is too little. The too little forms the bottom of the envelope.

With an optimum spark, there is a range of fuel air mixtures that it can ignite. This forms the edges of the envelope and are called LEL (lower explosive limit) and UEL (upper explosive limit). As the spark becomes less optimum, the side of the envelope will come closer together. As the fuel/air mixture approaches the edge of the envelope, the bottom of the envelope has to be higher.

Anytime the spark and the air/fuel are inside the envelope, there is ignition, regardless of the color of the spark.

The spark needs to be bright blue.

Also, you need to use a spark tester, NOT an old plug. The spark tester really makes the coil work hard. That way you can see what it’s capable of.

This is what the tool looks like. Should be available in most auto parts stores.

I actually had this problem with an old Honda. Checking the spark with a similar method you used, I also noticed an orange spark. Since I didn’t see a grounding problem or problem with the voltage source for the coil, I suspected a weak coil. I replaced it, and got a blue spark back, along with a much improved engine performance.

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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.

This is why if you raise the compression ratio and the amount of fuel being delivered of an automobile engine it can get to the point where the stock coil can’t fire the spark plugs because the spark is being quenched. So you have to step up to a higher output coil such as a MSD or Mallory to overcome the quenching effect. This is why top fuel dragsters don’t use regular coils. Because the engine compression is so high and the amount of fuel being delivered is so much, magnetos are used to overcome the quenching effect on the spark plugs


The first thing I check is the color of the ignition spark if I suspect an ignition problem. I use a spark tester. If it’s not blue then there is a problem somewhere in the system. I use this for any gas engine…lawnmowers, chainsaws, generators and the like.

If you have a GM car with HEI (coil on top of cap) replace the $20 or less coil. Make sure the carbon center electrode is not broken and is present. There should be a spring too.

I tried this when I was young. (I got the shock of my life and as thought about it then), Very stupid.
A need set of wires, plugs, rotor, etc full tune-up, was a although not cheaper but not so arm numbing. :evileye:

I like the following for testing coil effectiveness. I had a wide open throttle miss, supercharged engine, and i was able to determine a coil was breaking down when I open up the gap rather large, 2 coils handled gap, one didn’t, changed it and fixed my problem.

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