Advisory: synchronize the carburetor

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Advisory: synchronize the carburetor
Schermer

Advisory: synchronize the carburetor

Advisory: synchronize the carburetor

Advisory: synchronize the carburetor

Advisory: synchronize the carburetor

5 pictures

Advisory: synchronize the carburetor
Schermer

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Step 1: Preliminary work: To adjust the carburettors, they must be freely accessible. So down with the tank…

Advisory: synchronize the carburetor
Schermer

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Step 2: … and away with the airbox and intake manifold, until all adjustment screws of the carburetor battery can be reached without any problems.

Advisory: synchronize the carburetor
Schermer

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Step 3: The vacuum hoses of the synchronous tester are connected directly to the upper connection of the carburetor battery.

Advisory: synchronize the carburetor
Schermer

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Step 4: The throttle cables must have play. Attachment of the cables to the abutment (above in the picture) and pulley to the throttle linkage.

Advisory: synchronize the carburetor
Schermer

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Step 5: Know where: Carburetor battery from behind with designation of the components important for synchronization.

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Advisory: synchronize the carburetor

Advisory: synchronize the carburetor
How to synchronize the carburetor

Does the engine run rough, vibrate and not perform properly? Then it could be that the individual cylinders run against each other rather than with each other. What is needed now is a clean carburetor adjustment. Exactly the right time to plug in a synchronous tester.

Franz Josef Schermer

12/02/2009

Carburettors have to be synchronized especially if the valve clearances have been checked and possibly readjusted in the course of major inspections. A work step that is usually necessary every 12,000 to 15,000 kilometers according to the factory specifications of the respective brands. In order to achieve the best possible result, the engine must run in a mechanically perfect condition: Among other things, the air filter should be cleaned and the spark plugs cleaned or new parts inserted.

IMPORTANT: Before starting work, obtain the model-specific setting values ​​and regulations for your machine. In this way, among other things, the position of the idle speed, mixture and air regulation adjustment screws can be precisely localized. A tip on how to get such documents: Ask the (friendly) brand dealer whether they can make copies of the workshop manual for you. A donation to the coffee cup almost always helps! And one more thing is important: Please DO NOT twist all screws and nuts on the carburettors of your machine that are marked with red or blue sealing wax (and no matter how small the drop is)!

A NOTICE: The engine must be running to synchronize the carburettors. It should therefore be a matter of course to ensure adequate ventilation when working in closed rooms with the engine running. Exhaust gases contain toxic and odorless carbon monoxide that can cause unconsciousness and death. Open all windows and doors! The safest way is to use an exhaust extraction system, which leads the poisonous gases into the open. If the workshop rooms cannot be ventilated, work outdoors.

Adjustment devices and tools

In order to adjust the synchronization of the cylinders, there are some test and adjustment devices in the accessories trade. We work with that “Boehm synchronous tester” (Info: Telephone 089/888591, www.boehm-synchrontester.de). To adjust the (spring-loaded) adjusting screws on the connecting levers of the carburettors, you need an appropriately long Phillips screwdriver. With some machines you can only get ahead with an offset screwdriver.

Synchronize carburetor

Aim of the work: The engine should follow “softer” run, respond better when accelerating and clean “out of the basement” pull through.

  1. Let the engine warm up for at least ten minutes, a test drive is better: Then you have a fresh feeling of how the engine is running and better comparison options after the adjustments. “Warm engine” means: The oil temperature should be 80 to 90 degrees.
  2. Most machines require the tank and side panels to be removed. Place the tank on a work stand to the side and above the motorcycle.
  3. If a vacuum fuel tap is installed: Establish a connection to the vacuum nozzle on the fuel tap with an extra hose and T-piece.

Alternatively: A small container in the form of a mini canister is included in the scope of delivery of the “Boehm synchronous tester” recommended by us. This is filled with fuel, hung above the machine and connected to the carburetor inlet using the hose supplied.

  1. Connect the vacuum hoses of the synchronous tester to the connection nozzles provided on the intake port or the carburetors (see photo show step 3).

    If no connectors are fitted as standard, you must access the threaded holes closed with screws: Unscrew the screws that screw in the threaded connectors supplied with the synchronous tester.

  2. Make sure that the throttle cables have a little play: 0.5 to one millimeter in the cable results in about three to five millimeters of free travel on the throttle grip (see photo show step 4).
  3. Start the motor. Set the idle speed at the central idle adjustment screw (see photo show step 5) to the prescribed value. In the case of three and four-cylinder engines, this is usually between 1200 and 1300 rpm.
  4. Briefly accelerate a few times and observe whether the idle speed drops to the prescribed value and remains stable when you press “Close the gas”.
  5. Now check the vacuum difference between the cylinders: it should not be more than 0.03 bar.

    If the vacuum difference between the cylinders is greater than 0.03 bar, screw in or out the adjusting screws on the carburetor linkage accordingly (see photo show step 5): This changes the position of the throttle valves (or throttle slide) of the individual carburettors.

  6. Every now and then give a little gas (to 3000 to 4000 rpm) and check that way…
    …whether the engine accepts gas cleanly and
    …the pointers of all vacuum clocks rise and fall as evenly as possible.

If the test pattern is correct, the motor is correctly synchronized. If this is not the case, the carburetor could be dirty or the intake manifolds may be porous. This allows the motor to draw in air. Another source of error sometimes lurks in a defective ignition electronics. In general, the following applies: The motor cannot be synchronized without a precise error analysis and elimination!

When dismantling the synchronous tester, including the hoses and connecting pieces, check whether the rubber caps of the standard nozzles are not porous. Unscrew the screw plugs of the connection holes “hand-tight” and place a sealing washer underneath, if this is provided as standard.

Literature tip: In the model-specific “Motorcycle repair guides” from Bucheli-Verlag the settings for carburetor and injection machines are included.

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