I have trouble getting a handle on this "lean runs hot" concept. Back in the day, when British twins were all the rage, blue pipes were commonplace. There was even a product, Blueaway, intended to address this common problem. The go-to explanation for the cause of the dreaded blues was "your mixture is too lean". I accepted that as gospel. I'm a little older now and tend to look into things a bit more.
When I attended Airframe & Powerplant school, we were taught that exhaust gas temperature (EGT) was highest at the ideal stoichiometric air/fuel ratio, about 14:1. That lesson was reinforced when I was in the air force. I was a reciprocating engine mechanic. Our training manuals discussed adjusting fuel mixture using EGT and explained how EGT is highest at the ideal air/fuel ratio.
But I also have real world experience with blued pipes, and a stuck valve or two, that seemed to be the result of lean mixtures.
Every time I see a comment on this forum suggesting that someone's air/fuel ratio is lean because the header pipe is blue, I find myself digging around on the internet in search of an answer to this mystery. I pretty much always come up with the same answer. The EGT is hottest at stoichiometric, and gets cooler the further you move away from stoichiometric in either direction (lean or rich). So I figured I would share the best example I found. It's clear and easy to understand, and has a nice graph included. I'm not sayin that the blues aren't the result of a lean, or a rich, mixture. I'm sayin I don't fully understand the phenomenon. I figured I would share the info and see what shakes out of the bushes.
This is a quote directly from the Aviation Stack Exchange:
"Exhaust Gas Temperature (EGT) provides pilots a way of monitoring the fuel/air mixture in the engine. It uses the stoichiometric mixture (where Fuel and Air are perfectly balanced so that there is no unburned fuel and no unburned oxygen at the end of the combustion event) as a reference: At this mixture the EGT is at its hottest ("Peak EGT"). Making the mixture richer or leaner will reduce the EGT, and all other mixture settings are described in terms of "Degrees Rich of Peak" or "Degrees Lean of Peak"."
This is a link to their site:
https://aviation.stackexchange.com/questions/21466/what-appears-on-egt-gauge-if-the-mixture-is-rich-or-lean
Sorry about the link not being active. If you want to go to the site, you have to copy and paste into your browser. I'm old and don't have the IT skills that you whippersnappers have.
Finally, I have included a nice graph from Lycomming (aircraft engine manufacturer) that clearly shows the relationship between AFR and EGT. If you fish around on the web you can also find similar graphs from other manufacturers, like Continental, etc.
I'm curious about your experience with AFR as it relates to your exhaust pipe, cylinder head temp (CHT), stuck exhaust valves, etc. Regarding the afterglow, I'm with MatchlessG11, my bet would be a catalyst. If that Dyna muffler has a catalyst the AFR achieved with a carburetor is most likely way too rich for the catalyst. Rich mixtures cause catalysts to run off-scale hot. Just a guess. I know I wouldn't want that goin on in my muffler. Get it fixed.