The vacuum inside carburetors is important for engine operation because a fuel becomes vaporized easier on lower pressure. For proper engine operation front and rear carburetors in block have to be synchronized, i.e. they must have the same vacuum level of -0,3 bar.
The vacuum is measured in intake manifolds through small pipes intended for that purpose. On these pipes there are rubber caps. On some Virago bikes one of these pipes can be used for controlling Air Induction System (AIS) or some other system and then for measuring the vacuum it is necessary to temporary remove a rubber pipe leading from intake manifold's pipe to AIS.
Barometers showing the vacuum from 0 to -1 bar should be connected to intake manifold's pipes. Then the engine should be started and run in idle mode (cca 1200 rpms). Between carburetors there is one screw which is used for tuning the throttle ratio between two carburetors. Using this screw one should equalize both carburetor to show the same vacuum of -0,3 bar.
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| two carburetors BDS34 (xv535) synchronization the vacuum equalization |
When engine works, moving pistons produce pressure pulsation. Barometers on input tubes usually have valves which can be used to reduce the input flow in instrument and to stabilize the pressure for easier reading.
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