the main thing i would wonder, is if you hit anything with it.
even a tennis ball or a stick can sometimes shear the flywheel key. and since the flywheel key affects timing, there is a chance that it's sparking at totally the wrong time. the flywheel key is put there so that rather than twist or break the crankshaft, the key will fail, saving the expensive parts.
to repair this, you don't need many tools or much experience, and the key itself is about a dollar.
if you need more details, i can give them, but the lowdown on the project is this:
locate the screws retaining the flywheel cover, there should be 3 of them, 11 mm wrench should fit. (you might have to take a couple of small parts off of the flywheel cover, as the designers find it a conveinient place to stick things.)
once the cover is off, you should see a round clutch, that only wants to turn one way. this has to come off, the easiest way to get it off, is to take a small round metal bar, and put it on one of the lobes, and tap with a hammer. (remember, counterclockwise to loosen)
once this comes off, there is a washer, remove it.
you will now be able to see the flywheel keyway, and you should be able to tell if it's sheared off.
if it is, GENTLY tap on one side of the flywheel, and then the other, continue tapping until it starts to rattle, and then it should lift straight up and off. if it does not, do not tap harder, there is other methods for removing it.
clean out the fragments of the old key, and put the flywheel back on, and slip the key into the little channel.
put the washer and the clutch back on, remembering to tap it clockwise to tighten it. (it doesn't have to be seriously tight, just good and snug, as the first time you pull it it will tighten up to the exact amount of torque it needs.)
this should help.
if i can, i will add pictures of this process.
here is a set of pictures that show the difference between a good key, and a sheared key. in the second picture, this key is sheared about 5 degrees, rendering the engine inoperable. (please excuse my terrible artistic skills)
the third picture is of an actual flywheel and key in good repair.
the fourth is a picture of the start clutch, showing where is safe to tap on it.
the fifth, if it allows me to post it, shows the flywheel cover.