ASK THE MXPERTS: MY CRAZY SPARK-PLUG LIFE

Dear MXA,

I worry about the spark plug on my Honda CRF450. I have never given a passing thought to changing it, yet on my 2007 Honda CR250 two-stroke I changed it all the time. What’s the deal?

Spark plugs on a two-stroke were subject to oil contamination and carbon build up as excess premix oil turned into charred carbon. Four-strokes don’t have this problem, but their plugs still wear out, mostly from gap erosion. What is gap erosion? The spark jumps the gap from the sharpest edge of the plug’s electrode to the opposing sharp edge of the ground strap. Unfortunately, every time the plug fires, it eats away at the electrode and ground strap. When the sharp edge is worn down, it becomes harder for the spark to jump the gap. So, it moves to the next closest sharp edge, which is, of course, farther away. As that edge gets eaten away, the spark has to jump to a new location. The erosion of the sharp edges is hard to see, but it exists. The center electrode will round before the ground strap. If your bike’s fuel/air mixture, timing and carburetion are correct, your spark plug should last until the electrode’s edges are rounded off. We would change to a new plug every six months.