Yep, bad spark plug. Champion is usually to ones that quit me all at once, especially the little ones used in chainsaws and 3.5-5 Hp engine.
About spark plugs.
I have a old briggs l-head 20 hp twin. Really a nice looking clean engine on a MTD tractor mower. I knew if I killed the engine after it had been setting for awhile when hot it would most likely not re-start. Choke, no choke, did not appear to be gas related.
Engine would start up fast cold and could operate the engine for about 30 min to hour, kill engine and no re-start, fast cranking. Long story short could install new spark plugs of the correct type and gap and it would be ok for awhile and require NEW plugs again after these had run for few days, then back to square one, new set again. Just replacing one would not get a go.
I could install the removed plugs into another engine and get a good run and start.Could see what appeared to be a good spark when cranking the engine.
I finally decided that I was going to fix that engine or scrape it for parts. It would not start right when I needed it to go back to work. Kinda acted like it wanted to be on unemployment or disability social security and I do not allow slackers hanging out close to me.
I check the valve lash as right on, did a cylinder balance test as good, engine would idle really good at slow rpms and ok at fast rpms,
I installed a new clone coil, type coil that has both high tension leads out of the same coil body and the engine has been ok ever since. Will now even run and start ok with the type of plugs I do not like, CHAMPION.
Sometimes a guy has too just start swapping parts, especially when electronic magnetos are in the loop.