Author Topic: Idle Air Optimization 1970 M130 US version  (Read 87 times)


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Idle Air Optimization 1970 M130 US version
« on: February 14, 2020, 16:17:51 »
When I settled on disabling my speedswitch/two-way switch timing retardation system because is was faulty, I then had to reset idle speed down from about 1100 RPM. I have an 061 distributor and its corresponding throttle body so I found vacuum measurements unhelpful trying to use the instructions for the iterative process of adjusting idle from the Technical Manual here. Using the split linkage approach I was successful in getting RPMs down to about 800.
Further adjustments will be needed as I am pretty certain I need to advance distributor another 2 to 5 degrees but I don't have tools needed to do that so I wrote up a procedure that I could share with my tech who will be making the ignition timing changes. If the distributor does indeed need to be further advanced the idle speed may again be too high so I am hopeful this procedure will help him. This split linkage approach for idle air to fuel worked for me but I wanted the forum to comment in case it could be improved.

Idle air screw adjustment process:
  • Disconnect the linkage to the throttle body on the intake manifold so that throttle body air and injection pump fuel can be increased separately to test idle mixture.
  • Screw idle air screw in to reduce air until engine stumbles.
  • Screw idle air screw out slowly until idle smooths and RPM increases.
  • Advance the injection pump very very slightly and observe. Engine should stumble. If not, screw idle air mixture screw in 1/2 turn at a time until engine stumbles when advancing injection pump. At this point idle air mixture is too rich.
  • Screw idle air mixture screw out 1/2 turn and then advance the injection pump very very slightly. Continue making 1/2 turns out on idle air mixture screw until when advancing the injection pump  engine RPM increases. At this point the idle mixture is too lean.
  • Screw in idle air mixture screw making small changes (1/8 of a turn or so) to reduce idle air. Continue making screw in small changes until engine stumbles when advancing injection pump which indicates you have the correct air/fuel mixture at idle. You can confirm by opening the throttle body air a little and engine should stumble. If engine RPMs increase then you have turned the idle air screw in too far.
  • If the resulting idle RPM is too high shut off engine and turn down injection pump idle richness or if RPM is too low increase injection pump idle richness. Then begin the idle air screw adjustment process again until you get the desired idle RPM.
I am sure that the more experienced members here may have a suggestion to modify this procedure. This worked for me as I turned down idle richness two clicks on the injection pump and repeated the Idle air screw process each time.  The engine idles and performs beautifully now.
1970 280 SE W108.018
M130.980 w/ US emissions
manual shift