Carburetor Synchronization on Honda BF40A/BF50A outboards

CAUTION!
Running the outboard motor without sufficient cooling water will damage the water pump and overheat the engine. Be sure that water flows from the cooling system indicator while the engine is running. If not, stop the engine and determine the cause of the problem.

Remove the engine cover.
Remove the 5 x 10 mm plugs and sealing washers from the intake manifold of each cylinder.
Attach the vacuum gauge adapters to each intake manifold plug hole and connect the vacuum gauge hoses to the adapters.
- Connect:
  - The No. 1 vacuum gauge hose to the No. 1 vacuum gauge adapter.
  - The No. 2 vacuum gauge hose to the No. 2 vacuum gauge adapter.
  - The No. 3 vacuum gauge hose to the No. 3 vacuum gauge adapter.
Attach an engine tachometer and restart the engine.
- Follow the tachometer manufacturer's instructions.
Run the outboard motor in an outboard test tank with the water at least 4 inches above the antiventilation plate.
Allow the engine to warm up to normal operating temperature (Approx. 10 minutes).
WARNING:
Engine exhaust contains poisonous carbon monoxide gas that can cause unconsciousness and death. If the engine must be running, make sure the area is well ventilated.
Check the idle speed.
Pinch the SE tubes using commercially available tube clips keeping away from tube protector.
Check the intake manifold vacuum difference between cylinders. Take maximum vacuum difference between each cylinder. It should be 20 mmHg (0.75 inHg) or less.
If the vacuum difference between the cylinders is not 20 mmHg (0.75 inHg) or less, adjust as follows.

Adjustment:

The No.3 carburetor is the synchronization base carburetor. This carburetor will not have a synchronization adjusting screw.
Adjust the carburetor to get the least amount of vacuum difference between the cylinders. As the manifold vacuum difference decreases, the idle speed will become more stable.
1. Turn the No.1 and No.2 adjusting screws so that the vacuum difference between each cylinder is 20 mmHg (0.75 inHg) or less.
2. After adjustment, check the idle speed and adjust if necessary by turning the NO.3 carburetor throttle stop screw.
3. Snap the throttle several times and allow the engine to return to idle. Check to be sure that the vacuum difference between each cylinder stays within 20 mmHg (0.75 inHg) or less. Readjust if necessary.
4. Stop the engine, remove the vacuum gauge, adapters, and tube clips.
5. Install the 5 x 10 mm plugs and sealing washers.