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?