I’m reasonably handy, but internal combustion engines are pretty much a closed book to me. Our lawnmower generally runs ok, but the spark plug gets fouled (fouled = covered with a black, sooty-looking deposit) really quickly. Some research I did seemed to indicate that bad air filters could cause this, but cleaning and replacing the air filter didn’t seem to make a difference.
Any small-engine Doper experts out there?
Any chance that it’s not the exact recommended plug type for that engine? Physically, a different spark plug might fit, but if it’s not the correct type you might see these kinds of issues.
It sounds like it’s running rich (too much fuel in proportion to air). If the air filter isn’t restricted, it pretty much has to be a carburetor problem.
Is the plug deposit tarry or oily? If so, you might have worn or damaged piston rings. If it really is dry and sooty, that’s from an over-rich mixture. If the air filter is new, check inside the carburetor throttle body for obstructions, like dead bugs or leaves. If it’s clear, you’ll need to dial back the high-speed mixture. Usually there are one or more adjustment screws on the carburetor for this purpose–the high-speed screw (if present) will be located towards the middle of the carb, while the low-speed screw will generally be towards either the air intake end or the manifold end. Some small-engine carbs use an adjustable jet needle to control the open-throttle mixture, which means you have to partially disassemble it to adjust it. While you’re at it, if there’s a needle, take a close look at it. Is it bent or broken?
That’s all I can think of at the moment.
Dry black soot on the plug indicates, as others have said, a rich mixture. So it is getting either too much fuel, or too little air.
A dirtly airfilter is a valid cause. other causes might include:
A partially soaked or leaking float in the carb bowl.
A sticking or malfunctioning choke or leaking starting enrichment circuit.
Incorrect jetting or mixture adjustment. Can be an issue at high altitude.
Thanks, folks! Yes, it’s a dry, sooty deposit. I’ll try poking around in the carburetor to see if I can find any obstruction. I’ll also check the manual for carb adjustment screws.
That’s a good one that I hadn’t thought of. Definitely look at this before you go screwing around with jetting.