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Why gap spark plugs? A quick overview.


What are spark plugs and why do they need to be gapped?

“He can check the spark plugs and see if the engine’s runnin’ rich,” said the woman behind the counter.

I’d never been to this shop before, even though I’d driven by it hundreds of times. I’d stopped because of the sign that read Certified Harley Mechanic. I was just a little too skittish to drive my bike all the way out to the local Harley-Davidson dealership because I was afraid of stalling.

The place looked rough, with its worn sign and wobbly handrail, but I took a deep breath and walked in anyway. The inside looked just as rough, with creaky floor boards and yellowish dog-eared posters on the walls. I stood and waited in the long and narrow empty room until a back door opened and a woman came out. “Can I help you?” she asked with a smile. I exhaled and relaxed.

I told her that my bike had almost stalled twice the day before while slowing to a stop. The first time, I thought it was just my imagination. The second time, it made me nervous.

That’s when she started talking all “spark-plug-and-rich-engine” on me. Spark plugs? Rich? It was like she was speaking French and I only knew English.

“I’m sorry,” I said, then hesitated. “I’m new to riding. I really don’t know how motorcycles work. It’s my first bike.” I felt self-conscious and awkward.

She grinned. “Aw, don’t you worry about it, hon. Most of the men don’t know how their bikes work neither.” Her comment boosted my confidence. “I don’t know when he could get to it, though. He’s backed up about two weeks,” she said, shaking her head. My shoulders drooped.

“Okay, no worries,” I said, turning to leave. “I’ll think about making an appointment. Thanks!” How much longer could, or should, I ride my bike? I thought. The sun was bright and the summer air was steamy. I just wanted to hop on my bike, feel the rush of the wind, and go. But was it safe?

“Well, you just wait here a minute and lemme talk to him; it shouldn’t take long for him to check that out,” and she disappeared back through the door.

A few minutes later, an aging mechanic followed her out, wiping his hands on a rag and wearing a Harley-Davidson baseball cap that looked much-loved: frayed and faded. His name was Gary, and he agreed to look at my spark plugs. I followed him outside and plunked down on the stairs while he walked around my bike and look it over.

He asked me all about the bike: model, age, mileage, and how the engine was running. I watched him unplug some cables, unscrew the spark plugs, and put them up close to his face to examine them. “I dunno if they’re gapped right; someone might have just put them in straight out of the box,” he said.

I started asking questions. What are spark plugs for? What does “gapped” mean? How do I know what the gaps are supposed to be? How would I check? (Oh, and since I’m here, how often should I change the oil, oh yeah, how do I even check the oil?)

Gary was very patient with my questions. He answered them all as he measured and adjusted the spark plugs, explaining that the gaps were too big. If a gap is too big or too small, it could cause your engine misfire, lose power, foul plugs, get poor mileage, and/or accelerate the wear-and-tear on the plugs. He gently tapped the little “hooks” on the end of the spark plugs to make the gaps smaller, remeasuring often as he went. Once he was satisfied and reinstalled them, I started the engine. It rumbled to life in that distinct Harley-Davidson engine sound: chug-a-chug-a-chug-a. He nodded his head, and I gave him a “thumbs up.”

Gary spent almost an hour with me but only charged me $10. While I was there, I ordered a new headlight bulb—a brighter one for better visibility—and left the shop feeling happy about my visit: I found a new shop for bike maintenance, met great people, and learned a lot. Since then, I haven’t had any problems with stalling and my bike chugs in a satisfying, “even” rumble.

Use a feeler gauge to measure the gap of a spark plug

Spark plug “feeler gauge”

Here are my key takeaways about spark plugs:

  • The purpose of spark plugs is to create a spark that ignites the fuel-air mixture inside the combustion chamber.
  • Spark plugs are screwed into the cylinder.
  • If the spark plugs are not gapped properly, your engine will misfire and run poorly.
  • Even if a package indicates the spark plugs have been pre-gapped, they should be checked (and probably) adjusted.
  • To find the proper gap size for your spark plugs, refer to your owner’s manual.
  • Use a tool called a “feeler gauge” to measure the gap.

Interested in learning more? Here are some additional resources for understanding spark plugs and gaps better:


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