TwinMax I Carburetor Synchronizer

BMW R1150GS Motronic

BMW R1150GS Motronic

Gérard, alias GMax, is passionate about motorbikes, mechanics and DIY. He patiently notes all his interventions on his bikes on his blog and has allowed us to resume here his article on the synchronisation of his BMW R1150GS. Don't hesitate to visit him.



"The following information is the result of personal research. There is no guarantee of results. I decline all responsibility for any damage or costs that may occur during modifications to your motorbike".

The initial setting includes the adjustment of the left throttle stop. This setting is often referred to as "taboo": Nobody wants to touch it, but what if they have to?

After extensive research on the web, unsatisfactory or unsuitable, I decided to make my own experience. The result obtained after dozens of measurements and tests is very positive, which now makes it possible to fine-tune the synchronisation which is an essential element for the proper functioning of this engine.
The initial adjustment of the throttle valve timing requires a good knowledge of mechanics and the use of quality tools. Even if the operation seems easy, you should allow enough time and patience as it can take several hours, at least the first time.
It is important to read and understand the following before starting to manipulate. The work itself is not complicated if the procedure is followed correctly.

Standard synchronisation is simpler to achieve, but it is still necessary to be meticulous. These settings have been tested on my motorbike, an R1150GS single ignition model 2000 (R21) and are subject to change.

However, a two-cylinder will never run as well as a four-cylinder, so it has a certain character. A good timing of the Flat engine is necessary to enjoy this fabulous engine. Vibrations, constant speed jerks (surging or KFR), although these are difficult to absorb, or unstable idling are symptoms reported by owners whose motorbike is poorly synchronized. In general, the synchronization is adjusted because the engine does not "run smoothly".



There are two possible settings.

1 - The initial setting

This only has to be done once - normally at the factory - or in the event of major changes to the injection process, e.g. when replacing an injector body or replacing the throttle valve potentiometer, or in the event of maladjustment due to wear of the stops etc.

It is necessary if the difference in adjustment of the air screws is more than 1 turn. It is rarely necessary and therefore the synchronisation of the throttle valves is often reproduced.

2 - Synchronization  

The use of the motorbike causes wear and tear which, over time, results in a desynchronisation of the cylinders. This desynchronisation, even slight, reveals an unsympathetic engine character that the rider will soon discover: vibrations, backfire, jolts at constant speed etc.

The synchronisation check is carried out at all revisions and does not pose any particular problems. The synchronisation is hardly ever out of sync.



The updated version of this document should be printed out and kept close at hand during handling. The valve clearances must be correctly set and the spark plugs must be new. A measurement of the compressions will rule out a serious fault.

The tools :

  • a "Twinmax" differential pressure gauge” ,
  • a set of flat spanners,
  • a short flat screwdriver,
  • a long flat screwdriver,
  • a universal pliers,
  • a pliers with curved noses,
  • a Torx 20 screwdriver,
  • a digital voltmeter (for the initial setting),

possibly one or two fans to refresh the motor during the tests. Caution: never let the engine overheat (stay at 6 bars maximum). Do not leave it running unattended!



If the initial setting does not need to be made, proceed to the "synchronisation" chapter.

The left throttle and its potentiometer being the reference, they should be left as they are and try to adjust only the right side.

The goal to be achieved in the initial setting is to have an idle speed of 1050 rpm only by adjusting the throttle stops, having the air screws set exactly -2 turns, and setting the left throttle potentiometer to its initial value.

A digital voltmeter of sufficient quality to measure voltages of the order of 200-500mV is required. This adjustment requires a lot of precision and care.

Left throttle adjustment

  • The potentiometer attached to the left throttle body has two functions: to give information about the throttle position, i.e. the engine load required, and to inform the ECU of the idle throttle position, which will enable it to manage deceleration.
  • Indeed, when the throttle is turned off, the ECU cuts off the injection to save fuel but restarts the injection just before idling so that the engine does not stall.
  • This controlled deceleration is determined by the voltage withheld at the idle position. For this reason, it is necessary to reset the computer when changing the throttle position or potentiometer.
  • When the right side stop adjustment does not allow a good balance, it is probably because the left throttle adjustment is also distorted. We check everything.
  • Switch off the ignition.
  • Remove the left and right throttle cable.
  • Connect the vacuum gauge on each side instead of the plugs.
  • Adjusting the air screws: Screw in carefully until they stop and then loosen by two turns (-2 turns). At the moment only the left side is being dealt with.
  • Put a wire on terminal n°1 of the connector of the throttle potentiometer housing. To do this use a thin wire and place it in notch N°1 of the connector.
  • Reconnect the connector.
  • Connect the voltmeter between the wire and the body ground (there is a small ground wire connected under the body).
  • Switch on the ignition without running the engine
  • Measure the voltage and note it down before changing it.
  • Switch off the ignition.
  • Loosen the lock nut of the right-hand stop screw and loosen the screw by one turn.
  • Loosen the lock nut of the left-hand stop screw and loosen the screw until the stop is free, the throttle closed only by spring force.
  • 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.
  • Turning the stop screw clockwise (to the right) pushes the throttle valve back (gives gas) and turns the potentiometer until the value of about 260mV is reached. This value will be corrected later. The important thing is to be at 1050-1100t/min.
  • I noticed that the idle stabilization is only done after about a minute and that if you set the reference voltage too low the idle will be too low. So we use the potentiometer in our case to adjust the throttle which will have a play of a few hundredths of a millimeter.
  • For your information, the stop screw has a pitch of 0.5mm and each turn reduces/increases the throttle clearance by about 0.05mm; it doesn't seem much but a quarter turn is noticeable on the TwinMax.
  • Lock the stop screw. The stop screw will no longer be touched and remains locked.
  • Continue to turn the potentiometer to raise the voltage to 350mV. The setting controlled with the Moditec or GS-911 with the 350mV voltage is exactly in the middle of the recommended range which is 320-380mV. The original setting of my machine was 315mV. The potentiometer mounted on a gasket tends to go back a bit, so you have to play around a bit to get the desired value.
  • Activate the throttle slightly and release it to check the correct return to the stop.
  • Lock the potentiometer.
  • Wait for the idle to stabilize for one minute to be sure of its value. After each change of this setting, the Motronic ECU memory must be reset (see paragraph on this subject).
  • Switch to the right throttle setting.

Adjusting the right throttle

  1. The left throttle being well adjusted we will adjust the right one. The stop screw is free beforehand.
  2. Start the engine The idling speed is already close to 1050 rpm, do not touch the air screws.
  3. Synchronise perfectly with the right throttle stop and lock the screw. This adjustment must be done with precision.
  4. Switch off the engine.
  5. Check that the right-hand throttle valve moves freely and is not jammed when closed, in which case there is a mechanical defect that must be compensated for by giving the left-hand throttle valve additional play - for example, an extra half turn at the stop screw -, repeat the adjustment of the potentiometer to the desired value. Re-adjust the right throttle valve. Replace the engine and check the quality of the adjustment.
  6. Adjust the engine speed as required with the air screws. Once the initial setting has been made, synchronisation must be carried out.



Synchronisation must be done with the motor hot. To avoid overheating, put a fan blowing on the cylinders and/or check the oil temperature which must not exceed 6 bars. If necessary, allow the engine to cool down.

The adjustment of the synchronization is done at low speed, from idle by the air screws then up to 2500t/min by playing on the length of the cables.

  • Check that both sides of the air bypass screw (large brass screw on the injector body) are working properly. Tighten the screw without forcing it to the stop (fragile) and then loosen it by 2 turns. If the screw is seized, it is necessary to remove it from its housing, clean it, then grease the O-ring with silicone and then carefully put it back into its housing. Never force this screw against the bottom, as this will damage the cone that receives it in the housing of the injector body.
  • Remove the cap of the cold start accelerator (choke, lever on the left handle), loosen the locknut and give the sheath a clearance of 1mm.
  • Remove the throttle cap (right-hand handle), loosen the locknut and give the sheath 1mm clearance.
  • Adjust the cables to the left and right throttle by acting on the turnbuckles and give a play of 2mm.
  • Connect the vacuum gauge on both sides by removing the cap of the vacuum tube under the injector body. Caution: Never leave this tube without connection (vacuum gauge or plug).
  • The Twinmax must first be zeroed as described in the manual and used at maximum sensitivity when measuring. A very small tolerance of about half a value on either side of the zero point is permissible for a good adjustment.
  • Start the engine.
  • Adjust the idle speed with the air bypass screw on each side to 1050 rpm while keeping perfect synchronisation. In principle, the basic setting of this screw is -2 turns.
  • Reduce the slack in the left throttle cable to its minimum, without moving the engine speed, by acting on the tensioner and locking it with the locknut. Do the same on the right side..
  • Adjust the throttle handle clearance leaving 0.5mm.
  • Adjust the choke clearance leaving 0.5mm.
  • Check the synchronisation by accelerating very gently - really gently - up to 2500 rpm and if necessary act on the tensioner at the right throttle. This setting is the most delicate to carry out and is the one that will limit the jolts at constant speed. The difficulty lies in the fact that the ropes must be tightened at the throttle, on each side at the same time, hence the high probability of having to do it several times to reach a good balance. Above 2500t/min the throttles open enough so that the incidence of a slight misalignment is negligible.
  • Switch off the engine
  • Unplug the TwinMax.
  • Seal the vacuum tubes under the injector bodies.
  • Replace the caps on the choke and throttle cables.

If a slam is heard in slow motion on either of the throttles, the corresponding cable does not have enough slack and prevents the throttle from resting on the stop. In this case, the synchronisation must be re-synchronised, taking care to slacken the cables beforehand.



  1. Switching off the contact
  2. Pull out fuse 5 for 30 seconds.
  3. Replace the fuse
  4. Switch on the ignition without starting the engine and fully activate the throttle three times..
  5. Switching off the contact

The plug N°3 (Motronic management) mounted on my machine is N. 6136 8366 282, yellow/gold. The coding sheet allows you to choose the appropriate injection programme according to petrol quality, atmospheric pressure etc.

If one drives without the plug, the ECU takes a standard programme ensuring a rather rich mixture to protect the engine but increasing consumption. The ECU with its catalytic converter system will in any case adjust the richness in acceptable proportions.

The dealer knows which plug to use. An initialization of the housing must be done after changing the coding plug.



  1. The vacuum tube plugs are in place.
  2. The voltmeter is disconnected.
  3. The bypass wire from the throttle body is removed.
  4. The choke and throttle cable clamps are locked and the protective rubbers in place.
  5. The left and right stops are blocked - check by hand.
  6. Put some varnish on the stops and one of the potentiometer screws.

Have a good trip !