Posted for "Christopher Zavatson" <Christopher.Zavatson@udlp.com>:

Marv, Lorn,
When I went through this same battle a few years ago, I also installed a spring
in the heat muff.  A year later when I had the exhaust off for an unrelated
issue, I noticed the spring was gouging the stainless steel tube.  Every place
the coil touched the tube it ground in a recess.  I removed the spring.  It
hadn't made much difference anyway.  What did make a big improvement was to
take the intake air from within the  engine compartment.  That brought the
temperature rise up substantially.  Although the temperature gauge only
indicates a 15 deg F rise it does enrichen the mixture enough to cause the
engine to run rough until leaned.  
This change, of course, prevents the use of the heat muff for cabin heat.
Chris Zavatson
N91CZ
L360 std