USING THE TwinMax TO SYNCHRONIZE THROTTLE BODIES ON THE R1100RT/R1150RT

By Bill Hawkins

(with contributions from G. McCurdy, Dick Frantz & Lee Sayers)

 Purpose 

To achieve equalized fuel/air delivery to the engine cylinders at idle and moderate throttle openings.

The result is to produce a smooth and more efficient running engine 

Theory Of Operation

The TwinMax is a battery operated electronic differential pressure sensor that measures the difference in pressure (vacuum) between two sources. A running engine generates vacuum in the intake manifold which draws in an amount of fuel/air charge. The amount of vacuum presented to the fuel mixing system (the inlet air passage, and the fuel injector) is controlled by the amount of opening of the throttling device (such as a slide, or a butterfly). Achieving an equal opening of multiple throttles leads to each cylinder drawing the same amount of charge. This results in a smooth and efficient running engine.  The two tubes leading from the TwinMax must attach to a vacuum source, DOWNSTREAM of the throttle plates. On the RT, the ports used to attach the fuel vapor recovery hoses also provide a convenient location for attaching the TwinMax tubes.

With the tubes attached and the engine running, the difference, if any, in the vacuum from the two sources is sensed by the TwinMax, and presented visually by a needle on the TwinMax’s meter face. If one throttle opens more than the other does, it will cause the needle to move toward the hose attached to that throttle.

Before operation, with the engine not running, the needle on the TwinMax meter must be centered while the hoses are attached to their respective vacuum source. This should be done at the highest sensitivity setting in order to achieve the most accurate zero setting. 

During the synchronization, with the engine running, the needle may vibrate, or oscillate, in time with the opening and closing of the intake valves depending on rpm. “Centering” the needle to achieve equalization consists of setting the throttle openings so as to cause the needle to swing (or oscillate) an equal distance to either side of the Zero Point on the meter. The most effective sensitivity setting to use during equalization is to achieve about a º inch to 3/8-inch total swing of the needle.

 Synchronization Processes

Synchronizing the RT Throttle BODIES is a two-step process. First the idle fuel/air mixtures are equalized, and then the intermediate speeds controlled by the throttle bodies (twist grip) are synchronized.  Normally, both the right and left side body panels are removed to gain access to the throttle bodies and the hose connections. Removing the left side panel may not be required if the idle synchronization is not bad and adjustment can be controlled by the right Air Bleed Screw and you can attach the hose to the TwinMax to the port. 

Idle adjustment is accomplished with the engine running at… idle. That means with the throttles closed.  Open throttle, or intermediate speed,  synchronization is accomplished by changing the operating length of the RIGHT SIDE throttle cable by use of the adjuster at its end at the throttle body. This adjustment may be accomplished at any STEADY rpm achieved by locking or holding the throttle grip opened in the counter clockwise direction. Traditionally, adjustment is done at 4000rpm. However, some folks feel that 2500rpm, or the speed achieved when both throttles are just opened produces the same, or very similar results.  If you like to travel at a given speed, note the rpm you cruise at and make the adjustment there, or at least check it at that speed as well.  This will minimize any vibration at your favored cruising speed.  Note that in the following text, 2500rpm is used as the reference.

Preparation 

Tools: TwinMax, with fresh battery, and with tubes sized for BMW’s throttle body vacuum ports. , Slot head screwdriver, 3mm Allen wrench, 10mm open end, needle-nose pliers, and room fan (optional).

 Remove both the left and right side body panels. Then warm the engine up with the “Choke Lever” closed until the engine runs smoothly. Then, ride the bike to achieve normal operating temperature (5 bars on the Rider Information Display).

After the warm-up, shut the bike off and place it on the centerstand. Remove the rubber vapor recovery tubes from the bottom side of each throttle body.  Place the TwinMax at the seat location so that you can view the meter’s face.  Insert the hoses from the TwinMax onto the exposed injector throttle body taps,  A-side to the left and B-side to the right. Make sure the TwinMax is securely positioned.  Its recommended you use a room fan in front of the bike to provide cooling so the bike won’t overheat.   Work quickly as the bike can overheat without adequate airflow.  Periodically glance at the Rider Information Display to make sure the temperature stays in an acceptable range.

STEP 1  Adjust Idle Screws  (for balance throttle body vacuum at idle)

Idle synchronization is accomplished by adjusting the large brass Air Adjustment Screws low in the throttle body with the throttles closed. The idle state of Closed Throttles  is confirmed by ensuring:

The twist grip is in the full forward position and there is slack in both throttle body cables.
The throttle flanges on the throttle pivots are bottomed to the throttle stop

Each throttle cable displays about º to 3/8-inch play when pushed to the side.

At idle, the fuel injectors deliver a constant amount of fuel for each cylinder firing, monitored by the setting of the Throttle Position Sensor (TPS). Adjusting the Air Adjustment screw varies the amount of vacuum allowed to pass to the idle circuit, or more graphically, to allow more air to be drawn past the closed main throttle.

Adjusting these screws brings about two effects: The idle air/fuel mixture is changed and the idle speed is changed. 

Due to the need to maintain a viable mixture, with the TPS set within acceptable values, idle speed somewhere in the range of 1000rpm, +/- 50rpm should be achieved during idle synchronization. If synchronization within these values cannot be achieved, most probably the TPS setting must be changed. That procedure is outside the purview of this document, and it is suggested you contact a competent BMW dealer.

Practically, an idle speed as high as 1200rpm usually can be achieved without concern. Much beyond this level causes a non-viable air/fuel mixture, and may lead to an intermittent condition that is best described a “sticky” throttle on closure, resulting in an occasional high idle speed of up to about 1500 to 1600rpm. 

1. Loosen the locknut on the cable adjuster on top of the right-side throttle body cable, because this is where you will make the part-throttle adjustment in STEP 2. DO NOT loosen and adjust the LEFT side throttle cable, as this will effect the validity of the TPS setting. 

2. Turn on the TwinMax, and allow it to warm up for a few minutes.

3. Turn the SENSITIVITY knob fully to MAXI (full clockwise).

4. Adjust the ZERO knob until the meter needle is precisely through the middle of zero on the scale; this adjustment is very sensitive and very important.

5. Turn the SENSITIVITY knob back to MINI (full counter clockwise).

6. Start the engine and allow it to idle and again achieve normal operating temperate of 5 bars

7. Slowly turn the SENSITIVITY knob until a deviation shows on the scale. The less out of balance the two cylinders being tested are, the more you will have to turn the SENSITIVITY knob to show an imbalance on the scale. 

8. Adjust the brass idle/slow running screw(s) on the throttle body(s) until the meter shows zero. The meter is very sensitive, so care must be taken to use small increments of adjustment.  Optimally, you want to adjust each screw by an equal amount, but it’s possible to adjust only on one side.  Attempt to balance by using one side.  If you can’t accomplish the balance in º turn from one side, then adjust the other side as well.  

NOTE: The brass bypass screws are the only screws you need to turn for the Idle synching operation. ALSO, the screws may be dirty if the bike has 15k or so on it.  If so, Idle balance can’t be achieved. Shut off the engine.  Take the screws out and clean both of them and their seats with a cleaner suitable for fuel injection manifolds.  Reseat them, and back out about 1 turn.  Then repeat the operation from 6.

9.  Leave the engine running, and proceed to Intermediate Throttle Synchronization.

STEP 2  Intermediate  Throttle Synchronization  (to adjust for equal cable pull) 

If the lock nut on knurled cable adjuster for the right side throttle body has not been loosened, do so now.

1. Start with the SENSITIVITY knob about the middle of its range.

2. Start the engine, open the throttle gently, and run the revs up to 3000 or 4000 rpm.  The needle will oscillate. Your goal is to have it oscillate an equal amount on each side of the zero position, and about º to 3/8 inch in total width.  If the needle movement is not pronounced, increase the SENSITIVITY.

3.  Using about 2500 rpm, hold or lock the throttle grip open (if available your Throttlemeister or similar device.), and adjust the knurled portion of the right side throttle body cable adjuster with your other hand. The meter needle will be oscillating.  Adjust the cable until you can achieve an equal deviation on each side of the zero point. Let the bike return to idle (checking needle alignment again). If all is well, shut the bike off. 

3. After synchronizing at part throttle, tighten the locknut on the cable adjuster, making sure that the adjuster does not rotate with the locknut and alter your adjustment. It is easier if you hold the knurled adjuster with the set of needle-nose pliers while you tighten the locknut with a 10mm wrench to prevent slippage.  Re-check that the TwinMax needle is still zeroed at the chosen intermediate rpm after tightening to the cable lock nut and no change has taken place.

4.  Re-check your idle settings to ascertain that they have not changed.  You may need to work through the synchronization adjustments for both the Idle as well as Intermediate speeds to fine-tune the engine.

5. If all is well, turn off the bike. Turn off the TwinMax. Smile about a job well done.

6. Reinstall the body side panel(s) if you feel it looks better that way.