I've fitted the NHRC muffler, carbon fiber version. Excellent fit. Excellent construction. I did have to shorten one of the aluminum spacers in a lathe to get enough threads to feel confident of mounting. Alternative would have been to substitute a bolt 5 mm longer. had the lathe handy, so took that route. Be prepared for it to burn off some manufacturing oil for the first few hours of operation, producing an odor. May need to richen mixture a bit as some popping on trailing throttle is noticed. This was noticed with only the exhaust mod, before changing airflow.
My jetting after exhaust and air flow mods is still a work in progress, though close. Very little advice on this subject on this forum so far...
