A tiny piece of dirt so small you can't see it with your naked eye in the right(wrong) place can cause all sorts of nasty carburetor issues. Even if its squeaky clean when you installed it there could be trash in the tank that gets in there my daughter has just gone through this on her Honda.
I've had some personal experience with the under the hood in the dark fireworks on cars I have owned and I have seen it on motorcycles. If you can't do it in the dark you can always put your hand on the wire.
Don't assume that a new spark plug is a good spark plug. I had a problem once with a Subaru that would miss badly under heavy load. The dealer spent weeks trying to track it down. They finally discovered that three of the newly installed plugs were bad.