Spark plugs aren't universal; you have to make sure that you buy one that fits your mower. ... If you no longer have the manual or the number has rubbed off, you can always remove the plug first and take it to AutoZone with you.
Besides, what happens if you put the wrong spark plug in your lawn mower?
Spark plugs that are gapped incorrectly can cause an engine to miss, or run erratically, especially during idle. The incorrect spark plug gaps can cause uneven firing of individual spark plugs and delay engine combustion; both of which can cause an engine to miss or idle erratically.
Hereof, how do I know which spark plug for my lawn mower? Spark plugs are typically located on the back or side of a lawn mower engine. Look for a short wire that's about 1/4-in. -thick leading to the top of a short, cylindrical protrusion from the engine. If you're not sure where the spark plug is, consult your owner's manual.
What is the difference between rc12yc and rc14yc spark plug? The higher number is a slightly hotter spark type plug. The RC14YC4 is now the standard plug for the 990/992 Generac engines. The 999 in the new 16-22kW Evolution (7000 series) units and older 20kW units are still using the cooler RC12YC4 plugs.
For most racing applications, you usually want the plug gap to be between 0.020 and 0.040 of an inch. Most engine builders seem to settle around 0.035 of an inch. Factors such as the type of ignition you run, cylinder heads, fuel and even timing can affect how much gap will work best for you.