Miata Mailing List: February 1994, Message #168
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   From: sling@dseg.ti.com (Steven Ling)
Subject: Idle speed adjustment
   Date: Thu, 10 Feb 1994 18:47:57 -0500

A while back I changed to a K&N air filter. Immediately after that, I noticed 
that when I would disengage the clutch and let off the throttle, that the idle 
speed would drop very low. The engine would stumble, overcompensate to a 
higher idle speed and then settle to about 800 rpm after a few seconds. 
Several people told me that this was a classic idle speed adjustment problem. 
I looked back to the Winter '91 Miata Magazine (p45) and the recommendation 
was to "adjust your idle to 900 rpm with the adjustment screw backed out 
(counter clockwise) as much as you can and still maintain a 900 rpm idle 
speed." I looked in the 1993 Workshop Manual (page F-76) and it gave a idle 
speed spec of 800 to 900 rpm. Armed with this information, I went exploring. I 
connected a tachometer to the IG- terminal on the diagnostic connector, 
started the engine and measured the idle speed for various positions of the 
idle speed screw, beginning with the screw all the way in (clockwise). Note, 
it's probably a needle valve that would be destroyed by turning it down hard
against the seat. 

             3.0+----#----+----#----+----+----+----+--o-+
                |    #    |    #    |    |    |    |  o |
             2.5+----#----+----#----+----+----+----+o---+
   idle         |    #    |    #    |    |    o    |    |
   screw     2.0+----#----+----#----+--o-+----+----+----+
   turns        |    #    |    # o  |    |    |    |    |
   CCW       1.5+----#----+--o-#----+----+----+----+----+
                |    #   o|    #    |    |    |    |    |
             1.0+----#-o--+----#----+----+----+----+----+
                |    # o  |    #    |    |    |    |    |
             0.5+----#-o--+----#----+----+----+----+----+
               750  800  850  900  950 1000 1050 1100 1150
                            idle speed, rpm      
   
The original setting was just over 0.75 turns at 820 rpm. I set it to 1.5 
turns at 880 rpm. This solved the problem with the idle speed dropping, and it 
also seemed to improve the throttle response slightly (probably the faster 
idle). My guess is that the K&N filter changed the idle speed just enough 
to be a problem. Who knows. I had fun. 
                                  [][][]
     Steve Ling                                Texas Instruments, Inc.
     sling@dseg.ti.com                         P.O. Box 655012, m/s 31
     214-995-1378                              Dallas, Texas  75265