Setting 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!
How to set the lost motion gap
- Switch on Ignition
- Open and Close the throttle fully using the throttle cam to position the stepper motor
- Slacken the cable locknut from the abutment bracket
- Slacken the cable adjusting nut from the abutment bracket
- Tighten the adjusting nut to tension throttle cable until an equal gap is obtained on each side of the thottle body
- Tighten locknut to contact abutment bracket
- Using two spanners tighten nuts against abutment bracket
- Ensure gap is still equal each side of the thottle level
- Swtch off ignition. Settings will not be remembered if you do not do this.
In 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). This is step 2 on the attached excerpt from the Rover Workshop Manual. When opening and closing the throttle as described in step 2, the ECU should reset the stepper motor at exactly 25 steps. This calibration/initialisation to 25 steps 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.
The method shown in step 2 is the ONLY way that this can be done. Truly it is. Any other method that you have been told or heard of is a complete and utter myth.
To be absolutely clear of what I am talking about >> if some well meaning person on a forum or social network or down the pub has told you that you need to pump the accelerator peddle rapidly, 5 to 10 times to initiate or reset or fix the stepper motor, this advice is complete balderdash!
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 procedure is really straight forward!
Follow the above process to the letter for setting the Lost Motion gap / accelerator cable correctly.
Anyone can do this! You just need a couple of 1/2″ / 13mm spanners and good eyeballs to check the gap. This is one of those jobs where a natty inspection camera is very handy!
It should absolutely be done if you change the stepper motor or remove the throttle body/inlet manifold/exhaust manifold, and of course if you change the accelerator cable.
But why should you adjust the Lost Motion Gap?
The SPi system on a Mini is essentially an electronic carburettor, but it can't perform solely on its electronic cleverness. It still has a physical side which is important too, setting up the linkages to be 'just so' with the stepper motor is part of that. The lost motion gap allows the ECU to move the stepper motor within a specific range without the throttle cable restricting it. 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.