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While many modern V-twin motorcycles have gone to fuel injection, Yamahas V-Star 1100 is carbureted. All carburetors need maintenance from time to time. Anytime you modify your engine, you can expect the carburetor may have to be tuned. Otherwise, Yamaha recommends you tune them every 4,000 miles. Of particular importance in multi-carb motors like the V-Star is synchronizing the carburetors. Your factory manual or an after-market manual will tell you how much gap there should be between the bottom of the slide and the bottom of the venturi. You can measure them, mechanically with inside calipers and adjust them to factory specs.
Mechanical Synchronization
Your V-Stars carburetors have a venturi that air flows through. The air is modulated by the carburetor slide. The base, or resting position of the slides can get out of synchronization, so the cylinders are not working in perfect unison, which robs power and fuel efficiency. Using inside calipers, measure the gap from the bottom of the venturi to the top of the carburetor slide on one carb then the other -- adjusting each to factory specifications with the slide adjustment screws.
Suggested Tools
To synch your carburetors mechanically, you may have to use inside or straight dividers to measure the gap. Theres not a lot of room to work and look. A dental mirror may be of help. Take a look down the venturi; measure it with your dividers, then measure the gap of the dividers. Then, adjust to factory specs. Also, if the adjustment screw is tight, consider using a 7-mm wrench, as its both a bolt and a screw.
Manometers
A manometer is a device used to measure vacuum pressure. Its a far superior tool to tune your carburetors. The reason is, mechanically synchronizing your carburetors doesnt necessarily synchronize the vacuum created in the venturi, which is what really matters: equal amounts of gas and air entering each cylinder at the same time. Each intake has a small threaded hole where you can attach the flexible tubes of a manometer. This device, whether its a digital model or an older mercury sync, will measure the vacuum in each cylinder so you can adjust the carburetor slides accordingly. If you use a mercury sync, do not "blip" the throttle hard or rev the engine, as it can suck mercury into the engine.
