15,000 miles on a set of plugs with a carb is a lot of miles. If this is a recent development, I think I would start with at least checking the gap on the plugs, I suspect the gap has grown larger. I would probably change at least the plugs out. Wouldn't hurt to look at the condition of the point contacts either.

Back in the days of leaded fuel, and point ignitions, tune ups were done regularly around 10,000 miles or less. The unleaded fuel helped extend the time frame, but few made it past 12,000 miles before they needed attention. Gene