Discussion on possible causes of and solutions to a fouled spark-plug. Demonstrates governor spring tension increase to raise operating speed.
The next thing you want to do is pull the spark plug out of it so Iím 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.
Itís quite clean, itís got some blackness to it but thereís no large deposits. Looking at the insulator around the electrode in the centre, thatís not cracked, thatís very important. Inspect the insulator, hereís the insulator on the outside. Thatís the electrode.

Sparkplug fouled by over-fuelling

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 thatís 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 weíll just check that with the feeler gauges. Itís pretty much a perfect fit for thirty-thou so thereís 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. 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, Iím burning a little too rich so Iím going to add a tiny bit of speed to the top end. There we go. I just bent it a tiny, tiny bit.
 Here we go: Iím just going to put in my spark-plug after giving it a quick check. Now you definitely may want to change your sparkplugs. Theyíre cheap and sometimes if you have a no-start/no-run problem it could be as simple as a dead plug. Itís such a common problem that if you still have a no-start/no-run problem: itís not a bad idea to go get a second new plug and make sure that the brand-new new plug that you just tried isnít also dead.
[Mower starts]
OK now Iíve run the machine for an hour after the tune up and Ė check it out. Itís starting to whiten. OK, itís obviously hotter over on this side, you can see that. Spark is hotter on that side but itís starting to whiten up which means somethingís worked. Either the tune-up or the raising of the operating speed has leaned out the mixture enough that weíre starting to get a nice clean burn.


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