Dynobob wrote on 12/31/69 at 16:00:09:Aftermarket exhaust will be louder (some louder than others) and will backfire more (and blue the exhaust) unless you rejet the carb. With a rejet, backfiring will be minimal and the bike will run a whole lot better.
And allow me to clarify a bit on that for someone new...
The simplistic nutshell version:
The increased backfiring wouldn't be because the muffler messed up the system. Basically, the bike has too little fuel being mixed with the air in the first place (stock). When you improve airflow by upgrading the input (air filter) and/or output (exhaust), the original problem is accentuated. Rejetting, adjusting the idle mixture, and doing the white spacer mod improve the fuel flow.
If I misspoke here, someone please correct me.