I have the same problem every year. It was the valve adjustment. Look at the last section of the owners manual and they show how to gap the valves. You'll need a feeler gage and a socket or box wrench.
Take off the plastic cover (3 screws), You'll need a Torx for the rear one, and also to remove the valve cover screw. Be careful lifting the valve cover to try and keep the gasket intact.
Remove the spark plug and then slowly pull the starter cord to make sure the valve you are adjusting is fully closed. You can tell by looking at the other valve to see that it's fully open or pressed down.
Use your feeler gage between the rocker arm and valve stem and there is probably a lot of play. Gently tighten the nut in the center of the rocker arm a little at a time, moving the feeler gage until there is a bit of "pinch".
Then pull the starter enough to get the other valve fully closed and do the same.
You can try and start it with the valve cover off to see if your adjustment worked.
Once I get the feeler gage setting to get me in the ballpark, I start it up and warm the engine a bit, then at full throttle and using a 1/4" drive socket, I'll tweak the adjustment nuts tight or loose. The engine will start to die if I go too much either way so I find the point where the engine goes as fast as possible. I do the same with the other valve, put a drop of locktite at the top of the nut and put it all back together.
After about 8 years, the exhaust valve spring is relaxing and not fully closing the valve. I found springs online for $3.27 each. Pretty cheap to keep it running.