I have a one year old Craftsman lawn mower. Recently, the spark plug has become fouled with black soot after about 20 minutes of running. The machine starts to miss, and then stalls altogether. Cleaning the plug gives me another 20 minutes. There is no visible smoke, either white or blue. It does not use oil. I really don’t think the problem is with worn rings. Does anyone have an idea on what might be causing this problem?
How old is the gas?
My first guesses would be old gas or a stuck choke/a misaligned throttle cable (if you have a start position on the throttle lever and don’t have to push a bulb on the carburetor.)
It might also be a bad plug. You might just try another. For what it’s worth, my vote would be for any name brand besides Champion.
Sounds like the mixture is wrong, see here spark plugs speak
In every Haynes manual I’ve had there was a page just like this telling you how to read your plugs.
Sounds like fuel fouling, from an overly rich mixture (too much gas running through the engine). Choking sticking on, as mentioned, is a prime suspect. Other carburetor malfunctions can also cause it. But if you don’t find an obvious cause on first inspection, I agree it couldn’t hurt to try a new plug before taking it in for service.
There is a screw that controls the mixture on the side of the carburetor . It usually is a flathead screw, and has a spring on the shaft of the screw. You should be able to adjust the mixture by running the engine at “mowing speed”, and slowly turning the screw clockwise until the speed of the engine starts to cycle up and down every second or two. Then turn the screw the opposite way just a quarter turn or so past where the engine maintains a even speed. Then you should be set.
Be careful not to tighten the screw to where it gets hard to turn, as this will damage the carburetor, and cause it to run very rich all of the time. As long as you go nice and easy, it should be fine.
Mine did the same thing. It was obviously a fuel/air thing.
I started by removing the air cleaner/filter thing & banged all the fluff & dead grass & junk off it (I had last mowed a bunch of dead grass which got sucked into the aircleaner & jammed it up.
Ran like new after I cleaned that filter.
I would go with Inigo’s plan first, and then my suggestion only afterwards. It could very well be a plugged up airfilter, and it’s a good idea to clean it anyway.