Manual - Chapter 5 - Fuel & Oil

21: Recommended fuel: general

Note: The information contained in this Section and in the Specifications Section of this Chapter is correct at the time of writing. For updated information, or for more specific details, refer either to the rider's handbook supplied with the machine or to a local BMW dealer or other BMW importer.

1. At one time choosing the fuel tor a machine was a simple task, the main criterion being that of price. However, with the introduction tor environmental and health reasons of unleaded fuel and the progressive lowering of permissible levels of lead in leaded fuels, the situation is more complicated. Modern engines are also much more sensitive to the octane rating of the fuel used as they become more and more finely tuned to meet the conflicting demands of the greater performance and economy demanded by the consumer and the reduced pollution levels demanded by legislation. This Section expands on the basic information given in the Specifications Section of this Chapter.
2. First note that all recommendations are the minimum required. Depending on the quality of fuel locally available, on the operating conditions, on its owner's riding style or on its engine's particular characteristics or condition, any motorcycle may perform poorly on the specified grade of fuel and may require a higher grade to achieve normal performance.
3. Secondly, note that BMW advise against the use of any additives such as upper cylinder lubricants, octane boosters etc. Owners of machines used in the US should note that pure gasoline only is recommended - fuels containing a percentage of alcohol must not be used since alcohol will cause corrosion in aluminium, brass, rubber and plastic components and can cause severe engine damage. It may also cause bad starting and performance problems such as misfires or erratic idling.
4. Unleaded fuels should be used only as recommended. It is generally believed that the continuous use of unleaded fuels can cause accelerated wear of conventional valve seats, particularly on the exhaust; BMW have therefore fitted toughened exhaust valve seats to all 75 models and to all later 100 models (see below). However, modified engines (100 models only) can be identified only by reference to the frame number (where details are available) or by stripping the cylinder head to check the seats, so great care is required. Note that on suitably-modified engines it is preferable to use unleaded fuel rather than a low-leaded fuel.
5. At any time, if problems such as pinking (knocking) are experienced which could be attributed to poor quality fuel, attempt to solve the problem by changing the fuel before looking for a fault in the machine. First of all check carefully that the octane rating of the fuel used complies with BMW's minimum recommendations (see Specifications) then try higher grades and different types of fuel (eg leaded instead of unleaded) as well as different brands. Sometimes a cure can be effected by changing to a different filling station. If the fault persists, seek the advice of a good BMW dealer. Above all do not resort to unwarranted modifications to the machine; while, for example, it may be considered acceptable practice on many modern machines to retard the ignition timing by a small amount to compensate for reduced octane levels, this must not be attempted on the machines described in this Manual due to the difficulty of carrying out the task accurately without the special equipment necessary, and due to the fact that the effect of any such modifications on the machine's ignition and fuel injection systems cannot be checked.
6. Leaded fuel must be used on early 100 models and may be used on all ether models. From early in 1985 (from frame numbers UK K100 0 007 291, UK K100 RS O 081 107, UK K100 RT O 024 999 onwards, equivalent US information not available) all engines were fitted with toughened exhaust valve seats. These valve seats, which can be identified by their having a groove 1 mm wide and 0.2 mm deep machined around the inside diameter of the exhaust port side of each seat, are now the only type which will be supplied as replacement parts (or in replacement cylinder heads and engine assemblies) and so may also be found on earlier models. Note: BMW state that unleaded (premium grade) fuel may be used, if desired, on these early UK machines, but only provided that every third tankful is leaded fuel to protect the valve seats; owners of early US models should seek the advice of a local BMW dealer or the BMW importer tor confirmation of this.
7. Unleaded fuel may be used on all 100 models after the frame numbers given above (US owners must check with their BMW dealer or importer tor the necessary information) or on earlier models which are known to have been fitted with the toughened exhaust valve seats, and on all 75 models.
8. Premium-grade fuel must be used on all early 100 models (see details in paragraph 6), on all UK 75 models and on early US 75 models. It may be used also on later 100 and US 75 models.
9. Premium-grade fuel must be used on all early 100 models (see details in paragraph 6), on all UK 75 models and on early US 75 models. It may be used also on later 100 and US 75 models.

Next Section: 22: Exhaust system: removal and refitting