Setting the Lost Motion Gap

 

Why and When should you adjust the Lost Motion Gap?

If you have an SPi Mini then setting the Lost Motion Gap, which is also known as setting your accelerator cable up correctly, is very important.
If you are having issues with your SPi it is essential to get the engine (and yourself) on a level playing field, which basically means making sure the basics of the SPi setup are all done (including pretty standard stuff like servicing).
 
Setting the Lost Motion Gap is one of those essential things and it gets mostly ignored, probably because in old-school carb Mini's it isn't a thing and the accelerator cable position isn't crucial other than when the cable breaks.
 
The Lost Motion Gap probably wont get checked from year to year or from owner to owner and as that happens it will start to have a detrimental effect to the many other problems that happen with age. Setting this up properly REALLY does matter to an SPi Mini!
 
Setting even gaps allows the stepper motor to regulate the airflow to the engine independently of the driver's throttle input. If has too much or too little space (gap) on either side then it will be restricted or could over-adjust the air flow which will result in sub-optimal fuelling. I would go as far as to say that setting this up correctly is the starting point for getting your ECU to fuel properly.
 
If you have replaced your throttle cable then following this procedure is a must. If your car is stalling on start or idling poorly then checking the gap is also a good starting place.

How to set the lost motion gap

  1. Switch on Ignition
    Connect MemsFCR to check that the Stepper Motor has initialised correctly.
  2. Open and Close the throttle fully using the throttle cam rod to position the stepper motor.
    You should hear the stepper motor whir when you do this. If it doesn't then you probably haven't open it fully.
    With MemsFCR connected when you complete this step, the Steps should read between 25 and 35. 
  3. Slacken the cable locknut from the abutment bracket with a 13mm spanner
  4. Slacken the cable adjusting nut from the abutment bracket
  5. Tighten the adjusting nut to tension throttle cable until an equal gap is obtained on each side of the throttle body
  6. Tighten locknut to contact abutment bracket with a 13mm spanner
  7. Using two 13mm spanners tighten nuts against abutment bracket
  8. Ensure gap is still equal each side of the throttle level
  9. Switch off ignition. Settings will not be remembered if you do not do this.

 

Setting the Lost Motion Gap you will initialise the all-important stepper motor which is also known as the Idle Air Control Valve (IACV or just IAC).

When opening and closing the throttle cam as described in step 2, the ECU should reset the stepper motor between 25 and 30 steps (a JDM with Air Con may be as high as 35), you can check this if you connect MemsFCR at Step 1 and check the Steps after you have completed Step 2.

This calibration/initialisation is very important so listen carefully when you do it.
If you don't hear the stepper reset (clicks/whirs) during the step 2 action, repeat the opening and closing process, ignition off, step 1 and then step 2.

Myth Busting

I can't begin to imagine how many SPi owners have heard that advice and slavishly pumped the accelerator pedal (“Did I do 5 or 6 times? maybe I should do 10 times to be sure?”), and after they've done falsely thinking “that's done, I can now fix the rest of the problems” and wondered why they can't.
 
That single piece of myth and legend has cost collective days/weeks/months for the Mini community. Please make it stop, it doesn't do anything, it just gives your right leg some momentary exercise!
The Lost Motion Gap Procedure is really straight forward!