Manual - Chapter 5 - Fuel & Oil

19: Mixture adjustment and checks using an exhaust gas analyser


  1. The tasks described in this Section all require the use of an exhaust gas analyser (CO meter). Specialised equipment of this sort is normally available only to the better dealers and we would have to advise owners to take their machines to such dealers for servicing or similar work to be carried out. However, exhaust gas analysers have been in widespread use in the car world for some time now and will become more and more commonly available as emission control legislation is tightened. Accordingly, full information is given below for those owners who can gain access to an analyser and who feel that they have the skill and knowledge to use it successfully.
  2. There are various types of analysers available; in all cases the manufacturer's instructions should be studied and followed with care. In general, all analysers are fitted with a water trap to remove condensation and filters to protect the unit's delicate internals; these must be cleaned out and the filter elements renewed (where applicable) before each testing session.
  3. Switch on the analyser and leave it to run for a few minutes until the indicated CO reading has stabilised at the background level; it may be possible to recalibrate the analyser so that it reads zero at this level, if not, it will be necessary to subtract the background level from the test levels to obtain true readings.
  4. If the test results are to be accurate, the engine must be in good mechanical order (check the compression and valve clearances), the exhaust system must be in good condition and free from leaks, the induction system must be free from leaks and the ignition system must be in good condition (eg ignition timing correct and spark plugs renewed or at least clean and correctly gapped). The throttle butterfly synchronisation must have been checked, also the adjusters of the throttle and choke control cables, and the idle speed.
  5. Take the machine on a journey of at least 10 minutes duration so that the engine is fully warmed up to normal operating temperature, then check that the choke lever is pressed fully into the 'O' (off, or closed) position.
  6. Insert the analyser's probe into the silencer outlet to a depth of approximately 30 cm (1 ft) so that fresh air cannot enter and upset the meter reading, then start the engine and allow it to idle. Secure the probe so that it cannot fall out.
  7. Wait a few minutes for the meter reading to stabilise. Note that on US models in particular the reading may indicate quite a high level on starting the engine, due to the action of the Evaporative Emission Control System. The reading can be considered accurate when it has remained steady for one or two minutes.
  8. With the analyser producing a steady, true reading of the CO level, proceed as described in the relevant sub-section below; do not forget to make allowance for the background level when taking readings. Note that several seconds may pass before any alteration to the induction system produces a corresponding change in the meter reading; always wait for a few minutes after making any alteration to ensure that the correct reading is obtained. Different meters will respond at varying speeds and levels of sensitivity; some skill and patience will be required to achieve an accurate result.

Idle speed mixture adjustment

  1. Connect the test equipment as described above. If the reading obtained is outside the range of 2 ± 0.5% CO, remove the radiator cover panels or fairing right-hand lower side panels (as necessary) and prise out the plug from the top of the front right-hand corner of the air filter housing.
  2. Insert a 5 mm Allen-key into the airflow meter bypass screw and rotate it until the setting is correct (see general notes above). Note that the screw should be turned in very small increments at a time; the setting is very sensitive.
  3. On completion of adjustment, refit the plug with its index mark pointing to the rear. Check the idle speed and throttle synchronisation before refitting the fairing components or radiator cover panels.

Checking the air filter element

  1. The condition of the air filter element can be checked by using the exhaust gas analyser. Connect the equipment and take a reading with the engine running at (no more than) two-thirds of lts rated speed, ie approximately 5500 - 5700 rpm, maintain the test speed only for as long as Is necessary to obtain a true reading. Stop the engine, remove the air filter element and repeat the test.
  2. If the second reading was significantly lower than the first, the element is too choked to be of further use and must be renewed, regardless of its apparent condition.
  3. Check that the element is securely refitted when the test is complete.

Checking for induction system air leaks

  1. If the idle speed is unstable, or if the engine stalls when the throttles are shut suddenly, first check the ignition timing, the idle mixture setting and the valve clearances. If the fault persists, it may be due to a leak in the induction system.
  2. BMW give details of a method of checking for air leaks which involves dropping very small quantities of fuel on to each joint in the system and noting the effect on the CO meter reading. If the reading increases slightly (or the idle speed increases) there is a leak at that point, which must be sealed by lightening the clamp screw or renewing the component, as necessary.
  3. Obviously this is an extremely dangerous procedure which requires great care in its execution to avoid any risk of fire. Be especially careful to keep drops of fuel away from electrical components and to wash off all surplus fuel as soon as the test is complete. Owners are advised not to attempt this work themselves but to take the machine to an authorised BMW dealer tor the work to be carried out.