Manual - Chapter 5 - Fuel & Oil

18: Fuel system: adjustments and checks

Fast idle speed (choke) control cable

  1. The throttle operating linkage is opened a small amount on starting by the application of a handlebar-mounted control lever. On K100 RS, K100 RT and K100 L T models remove the fairing left-hand knee pad to permit access to the components for adjusting
  2. Move the handlebar lever to the first detent position to engage the first stage of operation and check that the idle speed adjusting screw (see below) is raised by the specified distance from its stop, then push the lever fully across to the second position and check that the screw is raised by the specified distance for the second stage setting:
      75 models 100 models
    First stage ... 1.5 mm (0.06 in) 1.0 mm (0.04 in)
    Second stage ... 3.5 mm (0.14 in) 2.5 mm (0.10 in)

  3. If adjustment is required, slacken the control cable adjuster locknut and rotate the adjuster, as necessary. Re-check the setting and tighten the locknut securely. Check that some free play is present between the cable outer and the adjuster when the lever is returned fully to the off position; the usual amount is 0.5 -1.0 mm (0.02 -0.04 in).

Idle speed and throttle cable

  1. The engine idle speed is adjusted only with the engine fully warmed up to normal operating temperature (minimum coolant temperature of 85'C/185'F
  2. On K100 RS, K100 RT and K100 LT models remove the fairing left-hand knee pad and lower side panel (as desired) to gain access to the adjuster screw.
  3. Check that free play is present in the throttle cable at all handlebar positions; if not slacken the adjuster and/or re-route the control cable as necessary
  4. With the engine idling at normal operating temperature check the speed recorded by the tachometer (rev-counter). If the speed is outside the specified range turn the idle adjuster screw to correct it; the screw is threaded into an extension of the cable pulley behind number 1 throttle body (75 models) or number 2 throttle body (100 models).
  5. When the setting is correct, adjust the throttle cable so that there is 0.5-1.0 mm (0.02- 0.04 in) free play between the cable outer and the abutment at its lower end when the twistgrip is firmly closed; use the adjuster at the twistgrip end to achieve the correct setting, then refit the rubber cover to secure it.
  6. Finally, slowly open the twistgrip and check that a distinct click can be heard just off the idle position, ie just as all cable free play has been taken up and the throttle butterflies are beginning to open. If no click is heard, either the throttle position switch is incorrectly set (see Section 17) or the idle adjuster screw is tightened too far; in either case check the settings carefully before riding the machine.

Throttle butterfly synchronisation

  1. While this task can be carried out using a set of four dial-type vacuum gauges, unless these are of very good quality, with proper glycerine damping, they will not be sufficiently accurate. BMW specify the use of a mercury filled manometer (part number 13 0 700, with adapters 13 0 702 and 13 0 703) which is much more accurate but more difficult to use.
  2. Given these facts, and that synchronisation is likely to be so infrequently required, owners are strongly advised to take their machines to an authorised BMW dealer for this task to be carried out by an expert using the correct equipment. For those owners who have the skill and equipment to carry out such work themselves, proceed as described below.
  3. First ensure that the air filter element is clean and/or renewed and that it is securely fastened, also that there are no leaks in the induction system. Check the setting of the choke and throttle cables and that the idle speed is correct (see above). Check that the ignition system (particularly the spark plugs) is in good condition, that the valve clearances are correct and that the exhaust system is sound, with no leaks or damage. If the engine has covered a very high mileage carry out a compression test (see Chapter 1 - Carrying out a compression test) to ensure that it is in good condition. Note that the engine must be fully warmed up to normal operating temperature (minimum coolant temperature of 85'C/185'F) before the synchronisation is checked. On K100 RS, K100 RT and K100 LT models, remove the fairing left-hand knee pad and lower side panel to reach the throttle assembly.
  4. Note: This check merely corrects small differences, using the bypass screws, in intake vacuum caused by production tolerances and the different rates of wear of various components. It does not provide sufficient adjustment for synchronisation of the butterflies themselves, which should never be disturbed. These are set at the factory using special air flow-measuring equipment; if the butterfly settings are ever disturbed the complete assembly must be renewed.
  5. If using equipment other than BMW's own, note that a T-piece adaptor will be required to enable the pressure regulator vacuum hose to remain connected while the test is conducted. The vacuum switch fitted to early 100 models (if still installed) can be disconnected during the test since it becomes effective only above idle speed. The vacuum take-off points are covered by small rubber caps. It was found that the hoses of a high-quality vacuum gauge set could be connected to these take-off points, provided that they were secured by small plastic clips. The pressure regulator was connected using the T-piece from a car windscreen washer system and a short length of tubing.
  6. With the engine fully warmed up and the gauges or balancing device securely connected with no air leaks, start the engine and allow it to idle. Where damping adjustment is provided, set it so that the reading flutter is just eliminated but so that it can respond instantly to any small changes in pressure. If aftermarket gauges are being used, it is useful to swap them between different throttle assemblies to ensure that all are producing exactly the same reading; if there is any variation this must be accounted for during adjustment. If any one cylinder is significantly lower than the others, there may be an air leak in the induction system.
  7. All gauges (or mercury columns) must show exactly the same reading with the engine at idle speed. If adjustment is necessary, it is made by rotating the bypass screw set in the appropriate throttle body. Do not disturb the screws and locknuts linking the various throttle butterflies. When all cylinders are giving exactly the same reading, stop the engine and disconnect the equipment.

Idle mixture adjustment

  1. The fuel/air mixture ratio can only by adjusted at the airflow meter bypass screw, with an exhaust gas analyser being used to measure the level of carbon monoxide (CO) in the exhaust gases. Owners who do not have access to such equipment or the skill to use it should take their machines to an authorised BMW dealer for the work to be carried out. Those owners who have the equipment and wish to carry out the check themselves should proceed as described in Section 19 of this Chapter.

18.3 Fast idle speed setting is adjusted at control cable

18.8 Adjusting throttle cable free play

18.16 Throttle butterfly air bypass screws (A); vacuum gauge take-off points (B); idle speed adjusting screw (C) - do not disturb any other screws

18.17 Idle speed mixture can only be adjusted at airflow meter bypass screw