The 30-second answer: Pull the plug with a 13/16” or 5/8” plug socket and read the tip: light tan is healthy, black and dry is running rich, black and wet is fouled, white is running lean or hot. Clean a lightly fouled plug, set the gap to .028–.030” (check your manual), and reinstall hand-tight plus about a quarter turn. If it is corroded, oily, or old, just replace it — a plug runs a couple of dollars.
The spark plug is the cheapest part on your generator and the first thing I check on a hard-starting or rough-running engine. It also reads like a gauge: the color and condition of the tip tells you what the rest of the engine is doing. Here is how to pull it, read it, gap it, and put in a new one.
Getting the plug out
Let the engine cool, then pull the rubber boot off the plug — grip the boot, not the wire. Clear any dirt from around the base so nothing falls into the cylinder. Use a proper spark-plug socket (most small engines take 13/16” or 5/8”) and turn counterclockwise. If it is stubborn, a little penetrating oil and patience beats forcing it and stripping the threads in an aluminum head.
Reading the tip — what each color means
This is the part most people skip, and it is the most useful. A light tan or grayish tip means the engine is running right. Dry and sooty black means it is running rich — think a dirty air filter or a choke stuck partly closed. Wet and black/oily means fuel or oil fouling — worn rings or a flooding carb. Chalky white or blistered means it is running lean or too hot. And a tip with rounded electrodes and a wide gap is simply worn out. So the plug does double duty: it can be the problem, and it can point you to the real problem.
Setting the gap
A new plug is not always gapped for your engine, and an old one drifts wide as it wears. Use a cheap gap gauge and set it to your manual’s spec — most portable generators want .028 to .030 inch. Bend the ground electrode gently against the gauge until it drags lightly. Too small a gap gives a weak spark; too large and the spark may not jump under compression, which shows up as hard starting.
Installing the new plug
Thread the plug in by hand first — always — to be sure it is not cross-threading the soft aluminum head. Once it seats, snug it with the socket about a quarter to half turn past finger-tight. You are sealing it, not torquing a lug nut; over-tightening cracks the porcelain or galls the threads. Push the boot back on until it clicks.
How often to replace it
For a generator that runs occasionally, I replace the plug about once a season or every 100 hours, whichever comes first — and any time the engine is hard to start or running rough. It is two dollars of insurance. I also keep a spare plug in the bag with the machine, because the one time it strands you will be the middle of a storm with the stores closed.
If a new plug does not fix it
If you put in a fresh, correctly gapped plug and still have no spark or a weak one, the problem is upstream — usually the ignition coil (armature) or a bad kill switch wire grounding out. Check the air gap between the coil and flywheel (about .010–.014” on most engines) and inspect the kill wire for a chafe to ground before you condemn the coil.