While many modern V-twin motorcycles have gone to fuel injection, Yamaha's V-Star 1100 is carburetted. All carburettors need maintenance from time to time. Anytime you modify your engine, you can expect the carburettor 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 synchronising the carburettors. 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.
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Your V-Star's carburettors have a venturi that air flows through. The air is modulated by the carburettor slide. The base, or resting position of the slides can get out of synchronisation, 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 carburettor slide on one carb then the other -- adjusting each to factory specifications with the slide adjustment screws.
To sync your carburettors mechanically, you may have to use inside or straight dividers to measure the gap. There's 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 it's both a bolt and a screw.
A manometer is a device used to measure vacuum pressure. It's a far superior tool to tune your carburettors. The reason is, mechanically synchronising your carburettors doesn't necessarily synchronise 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 it's a digital model or an older mercury sync, will measure the vacuum in each cylinder so you can adjust the carburettor 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.
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