Once the odometer hit 130 miles, I went out and got a new spark plug. It's a good number, and also my old competition weight.
I digress... Since the original (likely stock) plug was an old Champion, I went with its modern equivalent: the EZ start 5851 chainsaw model. There was only one left in stock at one location here in town, and I managed to get it. Must have been the lucky number.
It made quite a difference in the ride. The firing sounds so much smoother, more consistent- with a pleasant tone akin to a miniature turbocharger sitting over the rear wheel. I also noticed an increase in throttle response. Another replacement I should have done long ago. Well worth the pocket change. I didn't realize the popping inconsistent sounds during coasting weren't attributable to the cobbled DIY exhaust. Instead, it was the ancient plug complaining about the workload. It feels like the vibration scaled down a bit, as well. Glad I checked the plug after the last ride. The color was good, but that plug electrode was ugly. Figures- I assume it's older than me, too.