Maximize Accuracy During Carb Synchronizing

The Problem

Synchronizing the carburetors of the V Star 1100 requires special tools and precise adjustment of the synchronizing screw. To simplify this process, some have advocated running a vacuum line from the intake manifold on the front cylinder to the compartment that holds the tool bag. This compartment already has a vacuum line from the intake manifold on the rear cylinder to the AIS (air-induction system) or, on motorcycles that have the AIS removed, a vacuum line that is terminated in the compartment. This creates a very convenient spot for hooking up a carburetor synchronizer.

However, as shown in Figure 1, the vacuum line from the rear-cylinder intake manifold to the AIS includes a part called a "nozzle." This part comes with all V Stars (Classic, Custom, and Silverado) from 2000 to 2004. If a synchronizer is connected to the lines in the tool-bag compartment, will this nozzle affect the reading?

Figure 1. Callout of "NOZZLE" in Exploded View of AIS

The Experiment

I tested the carburetor synchronization using a Twinmax in four conditions:

1. No restrictors used; Twinmax connected directly to the intake manifolds.

2. No restrictors used; Twinmax connected to the ends of the two vacuum lines.

3. Restrictors used; Twinmax connected directly to the intake manifolds.

4. Restrictors used; Twinmax connected to the ends of the two vacuum lines.

As shown in Figure 2, the readout on the Twinmax is a needle that indicates an imbalance by registering on either side of zero. I hooked the vacuum of the front cylinder to side A and the vacuum of the rear cylinder to side B. In each of the four experimental conditions, I let the motorcycle idle at about 1000 RPM and took a reading from the Twinmax. Differences between conditions where the vacuum lines were used and where the vacuum lines were not used indicate that the nozzle interferes with the reading. The results are shown in Table 1. Note that the red-tinted area indicates the needle reading, which fluctuated at engine idle from left to right.

Figure 2. Readout of Twinmax

Table 1. Results of Experiment

	Restrictors Used	Vacuum Lines Used	Twinmax Reading
1	NO	NO
2	NO	YES
3	YES	NO
4	YES	YES

Conclusions

The restrictors reduced the needle swing, so I highly recommend using them to take readings. There was a slight difference between the line and no-line conditions, so I recommend taking the reading directly at the intake manifolds for more precision. It appears that the nozzle does affect the reading, if only slightly. Another option is to purchase a nozzle and install it in the vacuum line from the front cylinder.