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I filled up my lawnmower from a gas can earlier today and spent about 15 minutes trying to pull start it. I've checked the oil level and color and both seem fine to my inexpert eye. On two separate occasions, it seemingly randomly started and ran for about 10 seconds or so, but I haven't been able to get a third. I had a similar problem on my motorcycle some years ago when the carburetor bowl would slowly fill over a few minutes, but would run dry (jets, gr) after about 30 seconds of running.

Since any engine should theoretically run with air, fuel, and spark, I started troubleshooting in that order.

  • Air: I've taken out the air filter and cleaned it (I'll replace it later if it ends up making a difference). Reinstalled air filter and tried to start again, no dice. I also tried just leaving the air filter completely off, but it still wouldn't start.
  • Fuel: I have read that it's common for a fuel filter to be at the root of an engine that won't start, so I sprayed a small amount of starting fluid into the air intake and tried to start it again. Not even a small pop. I realized a few minutes later that it's probably not a good idea to spray starting fluid into the intake. Maybe?
  • Spark: On a car, you could test spark by grounding the spark plug against the frame somehow, but I wasn't able to do this and attempt a pull start. The plug is... basically black, but the electrode doesn't appear to be melted away. I tried to clean this by spraying it with carb cleaner and rubbing it, but it didn't make much difference. I put some more starting fluid in the spark plug hole on the basis that if the spark plug was sparking, I should get at least a little bit of combustion (fuel and air having basically manually been added) even if everything else is borked. But no, still wouldn't start.

Based off of the above, it seems like the most likely / lowest-hanging-fruit is that the spark plug is too gummed up / nasty / old / evil to spark successfully and that replacing it should solve that corner of the combustion triangle. If that doesn't and start fluid still doesn't work, then there's probably no power going to the spark plug, which could be caused by a faulty plug wire or the stator/whatever-it's-called-on-a-lawnmower-that-generates-electricity.

My question is, simply: based off of the above conditions, does it make sense to replace the spark plug?

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  • For those that encounter a similar situation, replacing the spark plug got the lawn mower running, but it now only starts when cold, which is (apparently) usually a problem with the ignition coil.
    – William
    Apr 18, 2017 at 3:35

2 Answers 2

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Yes, replace the spark plug - just make sure it has the correct gap before you fit it. And, at worst you end up with a used spare...

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  • Does my troubleshooting seem more or less sane?
    – William
    Apr 16, 2017 at 6:05
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    Yes, it's fine. What you could do is to get someone to pull the cord while you hold the plug (use a bit of wood or insulated pliers to avoid a shock but that would tell you you have a spark even if you don't see it :) ). Was the plug wet when you removed it after trying to start it? if so, it was getting fuel - if not then check the fuel is getting through.
    – Solar Mike
    Apr 16, 2017 at 6:13
  • Oh, thanks! I'll try to start it again this morning and I'll be sure to see if it's wet. I think it was dry.
    – William
    Apr 16, 2017 at 14:55
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I would ground the outside of existing spark plug on the engine first, and verify that you can see a spark jump the exposed gap. Not the same as in a combustion chamber under pressure, but at least this way you establish that the magneto/ignition system is doing something.

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  • I tried this before posting, but the plug wire is too short to be able to hold it against the engine while pulling the starter. I tried clamping it in place, but wasn't able to find somewhere to clamp it to that it would make contact and I could see it.
    – William
    Apr 17, 2017 at 21:50
  • If you can't hold it sideways, then look for a bigger clamp. A jumper cable comes to mind, but awfully long. Perhaps a vice grip pliers with a light gauge jumper wire to a cooling fin on the head?
    – mongo
    Apr 18, 2017 at 7:03
  • Well, I ended up replacing the spark plug and the lawn mower now starts just fine when it's cold, so I'm calling that mystery solved. Supposedly the ignition coil being bad is a cause of it not starting when hot.
    – William
    Apr 18, 2017 at 14:52
  • Ignition coils can fail cold as well. Glad you got it running.
    – mongo
    Apr 18, 2017 at 15:55
  • Good to know. I've adjusted the spacing out a bit am going to see if that gets the mower going.
    – William
    Apr 18, 2017 at 20:04

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