When rebuilding the carburetors and ganging them back together, the final step before installing them back on to the motorcycle was to perform a bench synchronization of the four carburetors. The idea is to get each carburetor throttle valve opening the same amount when the throttle is opened as a starting point. Here is a photo showing two of the carburetors with a light shining behind them to show the balance of the throttle valve openings:
Once installed on the bike you will want to perform a vacuum synchronization of the carburetors to fine tune the balance of the four carburetors due to variations in each cylinder and how much fuel mixture they are able to pull in. Before getting to the vacuum synchronization though I warmed up the engine and performed an oil change. The bike will be running for a while in a stationary position and who knows when the oil was last changed.
As you can see the oil was quite dirty comparing the old and new oil filters:
With the oil changed I setup a fan to blow air across the engine and rigged up my gauge for the vacuum synchronization.
There are many different gauges that can be used to perform the vacuum synchronization. Many are in the form of liquid filled manometers or dial gauges. Each of these types need all four tubes or dial gauges to be calibrated against one of the carburetors before you can sychronize all four carburetors. I used the Morgan Carbtune which is similar to the liquid filled manometer, but it uses metal rods instead of liquid and doesn’t need to be calibrated with each use.
The following picture shows the setup for the vacuum synchronization. The Carbtune and four hoses are hanging from the left handlebar, the hoses are then connected to the associated vacuum port on the carburetor intake boots, the engine is running at ≈1,750 RPM, and a fan is running to cool the engine (not shown). The fuel tank is also removed and the engine running on the auxiliary fuel tank since it will be in the way when it is time to make the adjustments. Ignore the gauge readings for now and I’ll explain what happened next.
If using the liquid filled manometer, as shown in the service manual, a correctly adjusted carburetor has the steel balls in tubes 1 and 4 at the same level and those in tubes 2 and 3 also at the same level, but lower by one half ball diameter compared to tubes 1 and 4. It has been determined that this half ball difference is ≈2cm Hg difference in vacuum pressure.
As you can see in the photo above, my carburetors are no where near the proper adjustment. This could mean that I did a terrible job with the bench sync of the carburetors, but it is actually due to trying to synchronize the carburetors without cylinders 2 and 4 running properly. Once the engine was running properly it completely changed how much vacuum cylinders 2 and 4 were pulling as compared to cylinders 1 and 3.
So now it is time to adjust the throttle valve openings to balance the four carburetors. I purchased another special tool from Morgan that they call the Carbtool. This tool is the combination of an 11″ screwdriver and an 8mm socket wrench that will allow you tighten/loosen the lock nut and turn the adjuster screw in a tight space between the cylinder head, carburetors, and frame.
Here is a photo of one of the adjuster screws and lock nut and the Carbtool in place:
There are three adjusting screws. One as shown above between carburetors 1 and 2. The other two are between carburetors 2 – 3 and carburetors 3 – 4. Carburetor number 3’s throttle valve is adjusted by the idle control knob. The adjusting screw between carburetors 2 and 3 is blocked by the choke cable bracket, so once the engine is warmed up, this bracket needs to be temporarily removed.
The adjusting order is to synchronize carburetor 2 to carburetor 3, then adjust carburetor 1 to be a little higher than carburetor 2, and then adjust carburetor 4 to match carburetor 1. It might take some back and forth since each adjustment can affect the previous adjustments.
Not too bad.
After completing the adjustment, set the idle back to 1,100 RPM and remove all of the test equipment.
One more chance to listen to the engine after all of the work that has been done to date, Audio. (Open in Windows Media Player after clicking the link)
Next step will be to clean and inspect the fuel tank to be able to get it reinstalled and get closer to a first ride.