Balancing 2 BING 40mm carburetors on a BMW flat twin in 3 steps using the TwinMax carburetor synchronizer
Adjust idle speed
- Warm up the engine for a good 20 km if starting from cold, less if the engine is warm.
- Use a screwdriver to adjust the idle richness screw carb by carb.
- Disconnect the right-hand spark plug wire and plug in a spare spark plug, the base of which should be connected to the engine/chassis earth (to avoid shattering the high-voltage coils).
- Go to the left-hand side of the bike and start the engine, which should still be at operating temperature.
- Turn the idle richness screw "in" then "out" slowly, until you feel the idle speed stabilize, and listen to the engine.
- When maximum engine speed is reached for the cylinder being adjusted, turn back 1/8 turn (unscrew).
- With perfect richness on the left-hand side, stop the engine (turn off the ignition key to avoid snapping the HT coils).
- Repeat the same procedure on the right-hand side of the motor.
- If the richness is perfect on the right, stop the motor (turn off the ignition with the key to avoid snapping the HV coils).
- Reconnect the HV cables to the motor coils.
Setting the idle synchronisation
- Unscrew the vacuum screws located under the 2 carburetors (Bing CV 40mm for me).
- Connect the Twinmax's two tubes to the inlets previously plugged by the two vacuum screws
- Switch on the Twinmax and set the needle offset to "0" using the two buttons on the casing (button 1: offset / button 2: sensitivity). As the result of this step determines all subsequent settings, the needle must be centered at "0" (motor off) and the instrument sensitivity at max.
- Start the engine (which should still be at operating T°) and bring the instrument sensitivity to 3/4 of maximum (sufficient for good settings).
- With the idle speed set, the needle should move more or less to the right or left.
- Select ONE carburetor and adjust the idle speed screw to bring the engine speed up to around 800-900 rpm, even if this further upsets the synchronization.
- Act on the second carburetor and adjust its idle screw to bring the Twinmax needle to "0". The needle moves a little with the engine's pulsations - this is normal and makes the first attempts at synchronization adjustment a little difficult.
- If at the end of the idle synchronization the engine speed has stabilized at around 800rpm (not less!) then you've arrived!
- If not, go back to step 6 and raise the engine speed A LITTLE. This will obviously upset your timing, but by acting on the second carburetor you'll get back to the ideal setting from step 8.
- Turn off the ignition with the key (to avoid snapping the HT coils).
- If you don't plan to perform dynamic synchronization afterwards,
- DON'T FORGET to tighten the carburettor vacuum screws!
Set dynamic synchronization.
- Unscrew the vacuum screws located under the 2 carburetors (Bing CV 40mm for me).
- Connect the Twinmax's two tubes to the inlets previously plugged by the two vacuum screws
- Switch on the Twinmax and set the needle offset to "0" using the two buttons on the casing (button 1: offset / button 2: sensitivity). As the result of this step determines all subsequent settings, the needle must be centered at "0" (motor off) and the instrument sensitivity at max.
- Start the engine (which should still be at its operating T°C) and bring the instrument sensitivity to 3/4 of maximum (sufficient for good settings).
- Now that idle speed has been established, the needle MUST NOT deviate to the right or left, as you have just performed idle speed synchronization!
- At the stop on both carburetors, touch the throttle cables and find out which one is tighter.
- Enlist the help of a third party, who will accelerate slightly for you up to 1500 rpm (that's enough!), to tighten the 2 throttle cables a little.
- Using a pair of pliers, adjust the tension of the most taut cable (loosen the tension a little) until the Twinmax needle is perfectly aligned with "0".
- Switch off the ignition with the key (to avoid snapping the HV coils).
- DON'T FORGET to tighten the carburettor vacuum screws!
Article written by "Reiler" (thanks to him). Feel free to visit the forum, it's a real goldmine http://www.flat-twin-bmw.com/