A wimpy bad spark plug, even a new one sure can make a guy feel weak upstairs especially after flogging several other more important and costly time consuming things when a engine won't start or run correctly.
Changing a spark plug or plugs is one of the first things I no longer overlook due to what you seen or worse.
Had a recent experience with such and seems most times it's a champion plug.
Chainsaw was starting a running good, next day when i hauled it too the woodlot it would only briefly try to start then eventually no promise at all.
Set the saw aside and used my other saw rest of the day.
Tried to start the bad saw later and no go, acted like no getting enough fuel, maybe carb issue.
Next day on the workbench I briefly eyeballed the fuel line and was thinking maybe carb kit but stopped immediately and changed the little champion spark plug. Fired right up and ran great with new plug.
Little wimpy spark plugs (usually Champion) had made me feel really weak in some previous experiences with engines.