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I notice that when I remove the spark plug from my chainsaw the starter cord pulls easily. But when I install the spark plug, the cord is hard to pull.

Why is this?

closed as off-topic by isherwood, Ed Beal, bib, Johnny, keshlam Nov 3 '16 at 3:19

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The reason is that when you start the engine with the sparkplug in, you are compressing the air in the cylinder. When the engine is running, the previous ignition cycle provides enough power to do it, but when you are starting up the motor that power is provided by you. Additionally, compressing air requires a substantial amount of energy.

When the sparkplug is removed, the air can easily escape through the sparkplugs hole, and since no compression takes place, the piston simple blows the air out, and so you feel very little resistance.

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