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[Mgs] SU Carb damper springs

Subject: [Mgs] SU Carb damper springs
From: Bob Howard <>
Date: Mon, 14 Apr 2008 10:03:53 -0400
  More to last week's writings about damper springs:
  Jim Taylor, retired design engineer, wrote this for the Jaguar Club.

    A piston spring combined with the weight of the aluminium piston
provides the necessary downward force to maintain essentially a constant
pressure loss (depression) at the throat of the carburettor.  The springs
are initially color coded with paint for identification. The paint
disappears with time. Common piston springs are as follows:
        Code                                Force in Oz                  
        Blue                                2.5   oz                     
          2   5/8 inch
        Red                                4.5    oz                     
          2   5/8 inch
        Yellow                            8.0    oz                      
         2   3/4 inch
        Green                            12.0   oz                       
        3 inch
        Red & Green                  11.25 oz                            
   3    7/8 inch
   It is difficult to identify springs visually since ones with the same
load rating may have different free lenghts, different wire diameter, and
different number of coils.  The springs may also sag slightly with age
and use.  To identify and check for proper specification I have made
cylindrical weights from aluminium bar stock corresponding to the above
table.   I put the spring in a glass tube with the correct weight on top
of it and measure the compressed height. If the spring is too tall it
must be replaced. If it is not over 1/2" too short it  usually can be
stretched.  Springs can be brough back exactly to spec and matched with
each other.  A squirt from a can of paint will serve for future

   Jim's idea of using a weight and keeping the spring contained in a
glass tube is likely to be more accurate than the way I did it ( before I
lost my table of spring heights).  I marked a dowel with the height, then
held spring with dowel inside on a scale, pressed down on the spring with
the specified force and read the height off the marked dowel.
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