Suck, Squeeze, Bang, Blow! In more proper terminology for the four strokes of the four stroke cycle internal combustion engine:  Intake stroke, Compression stroke, Ignition (Power) stroke, and Exhaust stroke. I suspect you have heard that all it takes to make an engine run is fuel, air, compression, and ignition. That is true more or less, but there is more to it if you want that engine to run well.

            Many of the questions I get asked by e-mail, letter, or telephone are about engine performance. Most of those questions about performance boil down to questions about combustion—the controlled burning of an air fuel mixture to produce power in the motorcycle engine. Chemists will tell you that the ideal mixture of fuel and air is about 14.7 parts air to one part fuel. Such a mixture properly compressed and ignited at the right time will result in complete combustion and the maximum power extracted from the fuel.

            To put that simple concept in perspective, just cruising down the road at a mere 3,600 rpm means that the crankshaft is revolving at the rate of 60 revolutions per second. That means that those four strokes in the four stroke cycle—suck, squeeze bang and blow—are each happening 30 times per second. Roll the engine RPM higher and of course things are happening proportionately faster. In this dynamic environment obtaining that ideal mixture, properly compressing it, and igniting it at the proper time is easier said than done. In truth, engines are often designed to operate with slightly richer (more fuel) mixtures than a chemist would say are ideal. There are several reasons for this but the two most important reasons are that engines operate better if slightly rich than if slightly lean; and, a slightly rich mixture does little harm to the engine while a lean mixture might.

            Stated another way, the proper amount of fuel combined with the proper amount of fuel, properly compressed and ignited at the correct time is what you need. Let’s start with the simple first:  compression. Given the size of the combustion chamber, the bore, and the stroke, what your engine needs for proper compression is sound valves properly adjusted; good rings properly seated; and nothing overly worn or broken. Most modern BMW engines have been good and durable in this regard. Proper compression also depends on the proper amount of air being drawn into the cylinder on the intake stroke, but this issue is discussed in greater detail below.

            Next, ignition must happen at the proper time, every time. This can be a problem with the points equipped models. Coil condition, spark plug wire condition, and spark plug condition sometimes cause problems. BMW has used electronic ignition systems on all models since 1981 and other than aging of components the ignition systems are very reliable. Some ignition module failures occur, primarily if the module gets too hot. A heat sink paste is used between the module and its bracket to aid in the conduction of heat away from the module.

            Now to the meat of this column:  the proper amount of air mixed with the proper amount of fuel is needed for the engine to run properly. This is the root of most engine performance problems on BMW motorcycles. BMW motorcycles have come with two basic fuel delivery systems:  carburetors and fuel injection throttle bodies. Nearly all current models available for sale are equipped with computer controlled fuel injection systems.

            The flow of air for the air-fuel mixture is controlled by the throttle. We think in terms of the “throttle” being what you twist with your hand, but the actual throttle is what restricts and controls air flow into the intake tracts of the engine. In slide-type carburetors it is the carburetor throttle slide that opens or closes to allow more or less air in. With CV type carburetors it is the throttle plate in the carburetor that directly controls air induction, but the diaphragm actuated slide also controls air to a degree. More importantly the slide carries the fuel metering needle so fuel and air delivery is supposed to stay properly proportioned.

            With the fuel injection systems used on BMW motorcycles, the flow of air is directly controlled by the throttle plates in the throttle bodies. When the twist grip is turned a cable is pulled, and then, linked either by cables or mechanical linkages the throttle plates in each throttle body turn, opening more cross sectional area for air to flow through.

            Fuel is delivered to each intake tract by the carburetors or fuel injectors. In carburetors fuel is delivered through jets:  an idle jet, a needle jet, and a main jet. For cold starting additional fuel is delivered through an enrichener often called the choke—but it isn’t really a choke (air limiter). It is an enrichener (fuel adder). The idle jet and the main jet are fixed. They have openings of a fixed size. The needle jet however has a tapered fuel metering needle in its center, so as the needle is raised or lowered by actuating the throttle twist grip; the cross section of the opening through which fuel can flow changes. Idle mixture can be fine tuned by the adjustment of the idle adjustment screw. With fuel injection systems how much fuel is delivered each injection pulse is determined by the size of the opening in the fuel injector, the fuel pressure, and the length of time the injector is open.

            There are many possible reasons for the fuel-air mixture to be either too lean or too rich. Even if I could think of them all I couldn’t cover all possible reasons in one column, or even several columns. But there are several notable causes for an improper mixture. Bear in mind that some conditions vary with engine speed, so an engine could be rich at idle and lean at wide open throttle, or vice versa.

Mixture Too Rich

Engines with carburetors often run rich somewhere in their RPM range. Carburetors simply are not very precise at metering fuel to match air flow. Assuming the correct jets are installed in the carburetor, the two primary reasons I see for these engines running too rich are fuel levels too high in the float bowls or worn fuel metering needles or needle jets. Floats which have been damaged by alcohol fuels can absorb fuel, and then they get heavy and allow a high fuel level in the carburetor bowls. Improper setting of the floats is also a common problem. Sometimes dirt, grit, or gummed fuel may cause a float needle and seat to seep, allowing a fuel level which is too high.

            The steel needle inserted into the center of the needle jet, and the needle jet itself does wear. This enlarges the cross section of the opening for fuel in the jet, and causes too much fuel to be mixed with a given volume of air. A clogged air filter can also cause a rich running condition.

            Engines with fuel injection systems are less likely to run rich than are engines with carburetors. There are several reasons for a fuel injected engine to be too lean, but fewer reasons for it to run rich. Details vary from model to model, but the primary reasons for a fuel injected engine to be too rich include:  a throttle position sensor which is adjusted incorrectly, worn fuel injectors, faulty fuel pressure regulators, and faulty temperature sensors. Far down the list, but worth checking, are dirty air filters.

Mixture Too Lean:

With carburetors, lean mixtures are typically caused by fuel levels too low in the float bowls, partially clogged jets, partially blocked fuel filters, fuel filters that are too small to deliver the fuel required, clogged petcocks, and air leaks. Often an engine runs fine at idle but under performs or loses power at higher RPM or full throttle. When this happens you should suspect a fuel delivery problem. Check the petcocks and screens, any fuel filters, and the jets. Sometimes an engine runs rough at idle, or has a high idle, but runs properly at full throttle or high RPM. In these cases suspect an air leak and look for leaks at the rubber sleeves that mate the carburetors to the heads, at the throttle plate shafts, at any vacuum hoses, and at the enrichener gaskets. Also look for low fuel levels in the float bowls.

            Lean mixtures in fuel injected engines are all too common. It may be that slightly lean conditions are really no more common than slightly rich conditions, but since the engines have been designed to run nearly as lean as possible for air quality and fuel efficiency reasons, anything leaner is easy to notice.

            Dirty and partially clogged fuel injectors will cause a lean condition. A clogged fuel filter may allow the engine to run fine at low RPM but to run poorly at higher RPM and throttle openings. Improperly adjusted throttle position sensors are also often the culprit. Air leaks at worn throttle plate shafts are fairly common on Oilhead engines once they are past 40,000 or 50,000 miles. Air leaks where the throttle bodies are connected to the head(s) are also common causes of lean conditions.

Synchronization:

Engines with two or more cylinders need for the cylinders to be operating together, with the same amount of air and the same amount of fuel at any given time. The Boxer twins are particularly troublesome in this regard. The Boxer twins (except for the R1200C) have two carburetors or throttle bodies connected only by fairly long cables. When the twist grip is turned the throttle plates need to open the same amount, at the same time, for the engine to operate properly. On the carbureted Airheads, a little bit off is hard to notice. The fuel injected models are less forgiving. The Oilhead engines, for example, have a single throttle position sensor (TPS) attached to the left throttle body. The engine control unit uses the output from the TPS to know how wide open the left throttle plate is. The engine control unit has absolutely no way of knowing how far open the right throttle plate is. It assumes they are opened the same. If the cables are out of adjustment then they won’t be. The O2 sensor samples blended exhaust from both cylinders, and once the engine computer makes corrections based on the O2 sensor, one cylinder will be rich and one will be lean. This is a frequent cause of unusual and unwanted vibration in these engines.

            The three-and-four cylinder K models are usually better in this regard because the multiple throttle bodies are mechanically linked. Mechanical linkages are far less prone to getting out of adjustment than cables. In all cases, idle synchronization and mixture can be fine tuned using the brass air bleed screws in the throttle bodies. The idle stop screws on Oilheads and the linkage screws on K-bikes ought not be messed with unless you have a compelling reason to correct a defect, and certainly not unless you know precisely what you are doing, and why.

            The methods we use to synchronize the cylinders on Boxer engines are not as precise as we would like to think they are. We are seeking to equalize the air flow into the two cylinders. We measure and seek to equalize manifold vacuum as a surrogate for air flow because we can’t directly measure air flow. Manifold vacuum can vary, separate from air flow, due to unequal valve adjustment, carbon or other deposits on valve stems or the underside of valve heads, or air leaks. If the vacuum readings are identical, but vibration or rough running persists, you will need to determine what other factors may be altering air flow.

            There are other less common causes for mixture problems. In any case, you need to systematically determine whether it seems to be a problem of too much air or too little air; or one of too much or too little fuel. Once you have determined this you are in a position to consider the possibilities, and to methodically attack the problem.

Good Wrenching!