Bulent/John,
CZ, as in CZ motorcyle.  REALLY neat machines in the early '70s.  I did really well on these until the Jap bikes started getting really good.  Really great handling bikes (motocross), especially the 250's.  Sorry, I digress - just fond memories of my mispent youth.  Love of motorcycles kept me from airplanes until I was in my mid forties!

Anyway, mufflers and motorcycles did not mix well in those days.  We did use a spark arestor (sp?) for woods events that contained a spiral insert like you describe but it didn't seem to make the bikes quieter.  Motorcycles, cars and airplanes is probably why none of us hear well.

Ken Powell