lawnmower. i ran over a stump and my mower lost half power, now won't start. what did i do?

There are 3 things that damage the unit. in least to worst:

1.The blade is bent, Probable but easy to fix.

2. The key that times the spark is damaged, very probable since you lost power.

3. The shaft is also bent in addition to the first 2 items.

Check the shaft and blade first since that is easy and cheap to do. If the shaft is bent, replace the whole unit. (Get a new mower) If the shaft is not damaged then replace the key and go back to cutting the grass. Get a service and maintenance manual for your unit. The way I check for a bent shaft is to mark 2 straight lines across the bolt head from point to point and have 2 strings intersect above the point and then turn the blade and observe the center point. It should stay centered. I remove the spark plug and tip the mower so the carb is up.

check the blade. But since you ran over a stump i think you probably damaged the lawnmower beyond repair so i say you should start saving for a new lawnmower.

Jim W has the correct answer, to replace the flywheel key you have to remove the engine cover and the flywheel. To remove the flywheel you may need a puller, the nut is torqued to around 50 Ft lbs and needs to be retorqued when putting it back together. I would remove the spark plug while checking the blades and shaft for damage for safety. If the shaft isn't bent you will be back in business shortly.

Check out the blade to see if it's bent or broken. You may also have bent the motor shaft. Also check out the blade housing to see if it has been bent into the path of the blade. These are all things that can be hurt by running over big, hard objects.

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have you ever checked or replaced the oil? Do it in case you haven't any longer. achieveable water in gas, this would reason galloping or pulsing of the engine blowing gray/black smoke as rpms climb to established. in case you ran out of oil, fill it ,run it 5 to 10 minutes drain and top off. if it has a clear out, gas or oil and air ,exchange them and need you probably did no intense injury. no longer rather, those small engines can take lots. if it is common to turn it with the pull initiate or elec. starter that's in step with hazard O.ok. and in simple terms desires a radical cleansing. good success.

damaged the shaft and set the timing off a few degrees. save it for spare parts and get a new motor at a mower shop..

You ran over a stump with a LAWNmower dumbass. Go get a new one.

increase the throttle... that's gonna get it going