Agreed. You might check ALL the plugs.
Carb cleaner sprayed around gasket areas will reveal vacuum leaks. I dunno...I still remember guys calling them "quadra-junks"...I've never had one. But if your ignition is up to snuff, I'd suspect a carb issue.
Good luck.

cm


If you can't fix it with a hammer and screwdriver, you need a bigger hammer.
1965 Chevy C10