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Howdy,

I have a 12 year old Craftsman self propelled lawnmower with a 4.75HP Briggs and Stratton engine.
It’s had this annoying issue for many years where it seems like the spark plug isn’t firing when it should, but only on some revolutions, never all the time (which would cause it not to run, of course).
Recently I tried to tackle it again, replacing the spark plug (properly gapped), the ignition coil, carburetor diaphragm and gasket, and air filter. Ran SeaFoam through it (before changing the spark plug) and took off the ignition kill wire from the coil temporarily to verify the issue wasn’t from that wire. Although it now starts easier, the exact same problem still exists.
I posted this video on YouTube. For whatever reason, it is a lot easier to hear in person than on this video. It sounds like a spark didn’t come on a revolution, the engine slows down briefly, then on the next revolution the spark comes again and the engine speeds back up.
Help! The last thing I can think of to replace is the carb, but it just doesn’t seem like a fuel issue to me. But I’m out of other ideas and looking forward to yours!

To me it doesn't sound like a carb issue. being 12 years old I suspect that it is floating the valves. Sounds like a valve not sealing sputter to me.

Thanks so much, @ILENGINE !

That makes perfect sense. I’m guessing that’s somewhat involved to fix? I suppose I’ll just live with it if it is. It’s more annoying than really impactful (at least for now).

sounds ok, now cut the grass, if you ever have to take the coil off again, sand the two bosses that stick up from the block, it grounds on the surface area not the threads.