Gérard is passionate about motorcycles, mechanics and DIY. He patiently notes all his interventions on his motorcycles on his blog, and has authorized us to reproduce here his article on the synchronization of his BMW R1150GS.
"The following information is the result of personal research. There is no guarantee of results. I accept no responsibility for any damage or costs that may arise from modifications to your motorcycle."
The initial setting
Initial adjustment includes setting the left throttle stop. This adjustment is often referred to as a "taboo": nobody wants to touch it, but what if they have to?
After a lot of unsatisfactory or unsuitable research on the web, I decided to conduct my own experiment. The results obtained after dozens of measurements and tests are very positive, allowing me to fine-tune the synchronization, which is essential to the proper functioning of this engine.
Initial adjustment of throttle timing requires a good knowledge of mechanics and the use of quality tools. Even if the operation seems straightforward, you need to allow sufficient time and patience, as it can take several hours, at least the first time.
It's important to read and understand the following before starting. The work itself is not complicated if you follow the procedure correctly.
Standard synchronization is easier to achieve, but you still need to be meticulous. These settings have been tested on my bike, a single-ignition R1150GS model 2000 (R21), and are subject to change.
A two-cylinder engine will never run as smoothly as a four-cylinder, which is why it has a certain character. Good synchronization of the Flat engine is necessary to enjoy this fabulous motor.
Vibrations, surging at constant speed (although these are difficult to resolve), or an unstable idle are the symptoms reported by owners whose motorcycles are poorly synchronized. In general, synchronization is adjusted because the engine doesn't "run smoothly".
There are two possible settings.
The initial setting
It only needs to be carried out once - usually at the factory - or when major changes are made to the injection system, e.g. when replacing an injector body, or replacing the throttle potentiometer, or when the stops wear out, etc.
It is necessary if the air screws are set more than one turn apart. It is necessary if the difference in air screw adjustment is more than 1 turn. It is rarely necessary, and so it is the butterfly timing that is often reproduced.
As the motorcycle wears, the cylinders become out of sync with each other. This desynchronization, however slight, gives rise to an unpleasant engine character, which the rider will soon discover: vibrations, backfiring, jerking at constant speed, and so on.
Synchronization is checked at every overhaul, and poses no particular problem. Synchronization can be easily adjusted.
Preparing for synchronization
Print out the latest version of this document and keep it with you when handling the engine. Valve clearances must be correctly adjusted, and spark plugs must be new. A compression measurement will help rule out a serious fault.
Tools required :
- Twinmax" differential pressure gauge ,
- a set of open-end wrenches,
- a short flat-blade screwdriver,
- a large flathead screwdriver,
- universal pliers,
- bent-nose pliers,
- Torx 20 screwdriver,
- digital voltmeter (for initial adjustment),
if necessary, one or two fans to cool the motor during tests. Caution: never allow the motor to overheat (keep to 6 bars maximum). Do not leave it running unattended!
The initial setting
If you don't need to make the initial adjustment, skip to the "synchronization" chapter. As the left-hand throttle and its potentiometer are the reference, we should leave them as they are and try to adjust only the right-hand side.
The aim of the initial adjustment is to achieve an idle speed of 1050 rpm solely by adjusting the throttle stops, with the air screws set at exactly -2 turns, and to set the left throttle potentiometer to its initial value.
You'll need a digital voltmeter of sufficient quality to measure voltages of the order of 200-500mV. This adjustment requires great care and precision.
Left throttle adjustment
- The potentiometer mounted on the left throttle body has two functions: to provide information on throttle position, i.e. the required engine load, and to inform the ECU of the idle throttle position, enabling it to manage deceleration.
- In fact, when the throttle is cut, the ECU cuts fuel injection to save fuel, but restarts it just before idle so that the engine doesn't stall.
- This controlled deceleration is determined by the voltage held at the idle position. This is why the ECU must be reset whenever the throttle or potentiometer position is changed.
- If the right-side throttle stop is not set correctly, it's probably because the left-side throttle is also set incorrectly. Check everything.
- Switch off ignition.
- Remove left and right throttle cables.
- Connect the depressiometer on each side in place of the plugs.
- Adjust the air screws: screw in gently to the stop, then loosen by two turns (-2 turns). For the moment, we're only working on the left-hand side.
- Connect a wire to terminal 1 of the throttle potentiometer housing connector. To do this, use a thin wire (in this case, a thin, flat, horticultural wire) and place it in the connector's #1 notch.
- Reconnect connector.
- Connect voltmeter between wire and case ground (there is a small ground wire connected under the body).
- Switch on ignition without running engine
- Measure the voltage and record it for future reference.
- Switch off ignition.
- Unlock the locknut on the right-hand stop screw and loosen the screw by one turn.
- Unlock the locknut on the left-hand stop screw and loosen the screw until the stop is free, the throttle closed only by the force of the spring.
- Loosen the two Torx-20 screws on the potentiometer housing to allow it to rotate, and adjust by turning the housing slightly until the voltage begins to rise (approx. 5 to 7mV).
- Lock the potentiometer.
- Turn the stop screw clockwise (to the right), which pushes the throttle back and turns the potentiometer until the value of approx. 260mV is reached. This value will be corrected later. The important thing is to be at 1050-1100 rpm.
- I've noted that the idle speed stabilizes after about a minute, and that if you set the reference voltage too low, the idle speed will be too low. We therefore use the potentiometer in our case to adjust the throttle, which will have a play of a few hundredths of a millimetre.
- For your information, the stop screw has a pitch of 0.5mm and each turn reduces/increases the throttle clearance by about 0.05mm; this may not sound like much, but a quarter turn is noticeable on the depressiometer.
- Lock the stop screw. Do not touch this stop again, as it remains locked.
- Continue turning the potentiometer to raise the voltage to 350mV. The setting checked with the Moditec or GS-911 at 350mV is exactly in the middle of the recommended range of 320-380mV. My machine was originally set at 315mV. The potentiometer mounted on a gasket tends to move backwards a little, so you have to play around a bit to get to the desired value.
- Activate the throttle a little and release it to check correct return to the stop.
- Lock the potentiometer.
- Wait for the idle speed to stabilize for one minute to be sure of the value. The Motronic ECU memory must be reset after each change to this setting (see relevant paragraph).
- Move on to right-hand throttle setting.
Right throttle adjustment
- With the left throttle set correctly, we now adjust the right throttle. The stop screw is already free.
- Start the engine Idle speed is already close to 1050 rpm, so don't touch the air screws.
- Synchronize perfectly with the right-hand throttle stop and lock the screw. This adjustment must be made precisely.
- Switch off engine.
- Check that the right-hand throttle moves freely and is not jammed when closed, in which case there would be a mechanical fault which must be compensated for by giving the left-hand throttle extra play - for example, by giving the stop screw an extra half-turn - and set the potentiometer to the desired value again. Reset the right-hand throttle. Start the engine again and check the quality of the adjustment.
- If necessary, adjust engine speed using the air screws. Once the initial setting has been made, synchronization is required.
Synchronization must be carried out with the engine warm. To avoid overheating, use a blower fan on the cylinders and/or check the oil temperature, which should not exceed 6 bars. Allow to cool if necessary.
Synchronization is adjusted at low revs, from idle by means of the air screws, then up to 2500 rpm by adjusting the length of the cables.
- Check that the air bypass screw (large brass screw on injector body) is working properly on both sides. Tighten the screw as far as it will go (fragile), then loosen by 2 turns. If the screw is seized, remove it from its housing, clean it, grease the O-ring with silicone and then carefully replace it in its housing. Never force the screw all the way in, as this will damage the cone which receives it in the injector body housing.
- Remove the cold-start gas pedal cap (choke, left-hand handle), loosen the locknut and allow 1mm clearance on the sleeve.
- Remove the gas pedal cap (right-hand handle), loosen the locknut and allow 1mm of play in the sleeve.
- Adjust the left and right throttle cables by acting on the turnbuckles and allowing 2mm play.
- Connect the vacuum gauge on each side by removing the cap from the vacuum tube under the injector body. Caution: never leave this tube unconnected (depressiometer or plug).
- The Twinmax must first be set to zero as indicated in the manual, and then used at its maximum sensitivity when measuring. For correct adjustment, a very small tolerance of half a value on either side of zero is acceptable.
- Start the engine.
- Set the idle speed with the air bypass screw on each side at 1050 rpm, maintaining perfect synchro. In principle, the basic setting of this screw is -2 turns.
- Reduce left-hand throttle cable slack to its minimum, without moving the engine speed, by acting on the tensioner and locking with the locknut. Do the same on the right-hand side.
- Adjust throttle grip slack to 0.5mm.
- Adjust choke clearance to 0.5mm.
- Check synchronization by accelerating very gently - really gently - to 2500rpm and adjust the tensioner on the right throttle if necessary. This is the most delicate adjustment to make, and the one that will most limit jerking at constant speed. The difficulty lies in the fact that the cables have to be tensioned at the same time on both sides when the throttle is applied, which means you'll probably have to adjust them several times to achieve the right balance. Above 2,500 rpm, the throttle valves open sufficiently for the impact of a slight maladjustment to be negligible.
- Stop the engine
- Disconnect the vacuum gauge.
- Plug vacuum tubes under injector bodies.
- Replace choke and gas pedal cable caps.
If a clicking sound is heard at idle on either throttle, the corresponding cable has insufficient clearance and prevents the throttle from resting on the stop. In this case, re-synchronize the throttle, taking care to slacken the cables beforehand.
Motronic box initialization
- Switch off the ignition
- Pull out fuse N°5 for 30 seconds
- Replace fuse
- Switch on the ignition without starting the engine, and fully activate the throttle three times.
- Switch off ignition
The N°3 plug (Motronic management) fitted to my machine is N. 6136 8366 282, yellow/gold. The coding plug allows you to choose the appropriate injection program according to fuel quality, atmospheric pressure, etc.
If you're driving without the plug, the ECU uses a standard program to ensure a rather rich mixture to protect the engine but increase fuel consumption. In all cases, the ECU, with its catalytic converter system, sets a rich mixture in acceptable proportions.
The dealer knows which plug to use. After changing the coding plug, the unit must be initialized.
- Vacuum tube plugs in place.
- Voltmeter disconnected (if necessary).
- Throttle box bypass wire removed (if necessary).
- Choke and throttle cable clamps locked and protective rubbers in place.
- Left and right stops locked -check by hand.
- Apply a little varnish to the stops and one of the potentiometer screws.