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I spent some quality time trying to get a Toro mower started. It has a Kohler engine. Many, many pulls and no life.

It had gas leftover from last summer in it (8 months old) and the mower itself is ten years old. So that was working against me. It was professionally serviced 3 years ago. They cleaned the carburetor, changed the oil, and put in a new sparkplug. I only run it maybe 10 hours a year.

Today I cleaned the carburetor, but still no go. Then I removed the sparkplug (a champion RA8HC). I tested it with my multimeter and got 17k ohms between the terminal and contact. The internet says variously that around 5000 to 10000 ohms is good. I cleaned it the best I could and reinstalled it, and somehow magically I made it run! It ran fine for the hour or so it took to mow the lawn.

Now here is my question. I know I should have tried to use up that gas last summer -- it's gone now and I can use new gas next time I mow. (I use ethanol-free.) Do sparkplugs gradually get worse, and is the high resistance a sign it is "going bad?" Do I need to replace it at all? Or was the problem a dirty plug and bad gas, which got good enough once I had a clean plug?

nuggethead
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2 Answers2

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For a mower, if the spark plug works, I don't think I'd worry about it too much. While the resistance was a bit high, that it's able to run means it's in good enough shape for now. You may want to put it on your maintenance list. Personally, I can't ever remember changing a spark plug in a mower. They will usually run for quite some time before needing changed. "Some time" meaning years ... and years.

As for the gas, that may be a different story. Considering you were running ethanol-free fuel, that's a good thing. You really shouldn't have to worry about it year to year. Since it is ethanol-free, it won't absorb water at any great rate. I've heard tale of ethanol-free fuel lasting for years. Still, at the end of the year, it's best for the carburetor to be empty of fuel before storing it for the year. Since I started running the bowl dry in my small engines, I've had zero issues with starting them again in the spring. Easiest way to make it happen is to turn the fuel shut-off valve off, then start the mower and let it run until it dies.

Pᴀᴜʟsᴛᴇʀ2
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If a spark plug fails to "spark", it can often be fixed by cleaning its active side.

The spark of a spark plug (Image from https://commons.wikimedia.org/wiki/File:Bougie3.jpg, CC-BY)

On used spark plugs, the part where the spark is generated is often dirty and covered in carbon black. Clean that away. The above image shows how it should look like. The spark is generated between that thorn in the center and the metal bow.

Additionally, the distance between the two electrodes must be within a certain range, typically around 0.5-0.8mm (the exact range should be in the engine manual). If the distance is way bigger than that, it won't work correctly. This can be fixed with a few careful hits with a hammer on the bottom piece.

PMF
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