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Spark Plug Troubleshooting

Discussion on possible causes of and solutions to a fouled spark plug. Demonstrates governor spring tension increase to raise operating speed.


Transcript provided for the hearing impaired:

The next thing you want to do is pull the spark plug out of it. So I’m just going to pull the wire off, push it aside. Put a socket wrench on it nice and easy, spin it out and have a look. It’s quite clean, it’s got some blackness to it but there’s no large deposits. Looking at the insulator around the electrode in the centre, that’s not cracked, that’s very important. Inspect the insulator, here’s the insulator on the outside. That’s the electrode.

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This sparkplug is a little bit blackened, a little bit fouled, you would say. Now the most likely cause of this is overfuelling or a rich fuel condition. Reasons for a blackened or carbon deposited spark plug would include things like too rich a fuel mixture. Or a sticking choke or a clogged air cleaner. You know, anything that’s going to increase the fuel in the fuel : air ratio.

Also a good possibility is oil seeping past the rings on the compression piston. And getting up into the combustion chamber and fouling it. I had the good sense to hold onto the literature that came with the lawn mower when I bought it ten years ago. See it says right here the gap on the spark plug should be thirty-thou. So we’ll just check that with the feeler gauges.

It’s pretty much a perfect fit for thirty-thou so there’s nothing wrong with our gap. Clean it up with a nice soft wire brush and reuse it. Anyway, so there is no adjustment here. The only adjustment I can make is I can bend this metal tang… Right here that the governor spring is controlled by and that will speed up my high end.

Governor adjustment

This is the governor spring right here and this metal tang here is its anchor point. Stretch the spring further to speed it up or compress the spring a tiny bit to slow it down. In this case, I’m burning a little too rich. So I’m going to add a tiny bit of speed to the top end. There we go. I just bent it a tiny, tiny bit.

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Here we go: I’m just going to put in my spark-plug after giving it a quick check. Now you definitely may want to change your sparkplugs. They’re cheap and sometimes if you have a no-start/no-run problem it could be as simple as a dead plug. It’s such a common problem that if you still have a no-start/no-run problem… It’s not a bad idea to go get a second new plug. Make sure that the brand-new new plug that you just tried isn’t also dead.

[Mower starts]

OK now I’ve run the machine for an hour after the tune up and — check it out. It’s starting to whiten. OK, it’s obviously hotter over on this side, you can see that. Spark is hotter on that side but it’s starting to whiten up which means something’s worked. Either the tune-up or the raising of the operating speed has leaned out the mixture enough that we’re starting to get a nice clean burn.

Examine the spark plug!
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