A new spark plug does help in fixing a lot of ignition problems. Old spark plugs can sometimes be cleaned by soaking in a carburetor cleaner, do not use anything abrasive as small particles can end up in the cylinder where they cause damage.
The center electrode rounds off after awhile in service, increasing the voltage required to produce a good spark. Clean any pitted or dirty surface of the metal electrode. Next measure the gap on the electrode tip. Spark plugs are best gapped with a cam-type gauge, gently open or close the spark plug gap according to the correct specification for the type of spark plug and equipment it is fitted to.
The heat range of the spark plug depends primarily upon the exposure of the insulator to combustion heat. The greater the exposure and the longer the thermal path, the hotter the plug. The plug should run hot enough to prevent any fouling, but not so hot that it ignites the mixture early before the spark does. Warning Should this happen, the engine self-destructs!
Manufacturers describe heat ranges numerically. For domestic plugs, the higher the number, the hotter the plug. At the first sign of trouble, replace the spark plug with the same type and, whenever possible, with the same brand specified in the owner’s manual. Interchange guides list physically equivalent plugs, but heat ranges can vary somewhat between manufacturers.
If testing a different spark plug, begin with a cold plug and run the engine under normal workloads for an hour or so, adjusting the carburetor for best power. If the plug blackens and fouls up, go hotter one heat range. Continue a step at a time with spark plugs from the same manufacturer until the insulator takes on a dark brown color, indicating that temperatures are normal. Err on the side of cold (dark), since the worst that a cold plug can do, is foul up.