Is it possible to screw up changing spark plugs?

you could strip the threads...

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I changed the spark plugs in my mower last night and it started right up. Let it run a few seconds then turned it off. Tried starting again and nothing. It tries to turn over but just won’t start.

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I put the old ones back in (they worked before, just needed to be replaced). Still nothing.

Is it possible to frick up while replacing spark plugs? I have no idea what I did wrong.

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Does it have gas in it?

You didn't leave ethanol gas in the carb did you? It may not be the "fire" but rather the "fuel". Ever check to see if you're getting fire at the cylinder? Take out plug and keep the wire on it and ground it against the block. Have someone pull the rope and see what color fire the spark is. Works great in a darkened environment or dark.

Of course make sure you practice good safety and be careful for fumes in the cylinder so you don't catch things on fire.
Unless the coil went bad...that's really the only thing that could be on the ignition side of things (assuming it's a push mower)

you didn't do this, but you asked. Yes you can screw up spark plugs. Most common way is not to tighten enough and the cylinder pressure will literally blow them out of mower creating catastrophic failure.

Based on your situation. I'd say either you didn't attach the leads tight enough, or you attached them to the wrong plug. After that, I'd remove the air filter and try a bit of carb cleaner sprayed in the intake (like just a little, don't need much). If it starts off that its a fuel issue.

That’s comforting, thanks.

I'm still betting on the gas in the carb being your problem. You can have someone else turn it over and you watch the spark and I bet you'll have a pretty blue spark indicating a good coil/wire/plug combination.

I personally have been bit by the ethanol bug many times. Used Stabil. Still gummed it up. Used Star-tron and it did the same thing. Finally got smart. Put a valve in line between tank and carb....turned it off and ran carb dry...voila...every year after that no issues.

Yea but I don’t use any ethanol gas in it. It’s always been 100% gas.

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Yea it has gas. I’m afraid I may have really fricked up on this one.

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unless you stripped the threads, i don think you can "really frick this up"
you said the new plugs worked the night before. So to me it seems it should be a fule issue.... was the tank low? could have sucked debris and clogged the carb. have you tried using starter fluid into the air intake to see if it will turn over?

Find out what the gap is supposed to be and run down to autozone and grab one of these for .99 cents. Then set the gap to the appropriate setting.

You may also want to take the plug out, put the connector back on the plug and turn the engine over a few times to make sure it's sparking. Whatever you do... DO NOT hold the spark plug when doing this.

Mind the gap.

unplug the big square wire harness plug to the motor

if it starts then its one of your kill switches is the problem, maybe the blades are still engaged or its not in neutral or your foot is not on the brake

NOTE - when it starts you need to plug the plug back in order for the key to kill the engine