Aprilia Falco Synchronizing the Cylinder Vacuum

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Filed Under (Aprilia) by admin on 26-10-2010

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Motorcycle cylinders work quite independently compared to automobiles. Separate carburation, intake manifolds, exhaust pipes and sometimes even independent air filters allow the cylinders to be tuned so that one may be making more power than another. This can be due to differences in air flow, temperature, injection, or valve adjustment betwen the cylinders. Periodically, the cylinders should be synchronized. This is usually done by comparing intake manifold vacuum beneath each throttle and trimming the mixture until balanced. Some old-timers will tell you it can be done by ear, listening to the air flow in each carb throat through a tube stuck in your ear. Most modern tuners have switched to mercury sticks. Rigid tubes stuck in a bath of mercury are attached to the vacuum source. The vacuum draws the mercury up the sticks in proportion to pressure difference between the manifold and the atmosphere. In order to smooth out the individual vacuum pulses, a damping device is needed. This is nothing more than a small orifice (pin hole) restriction in the lines, placed close to the manifold with an air reservoir (length of tube) behind it. Because there is no real air flow in the gauges, there is no pressure drop across the orifice. But when the manifold vacuum drops there is a delay before the gauge pressure can bleed off and it appears steady and readable. Other types of vacuum gauges include mercury-less versions (that draw metal rods), or traditional needle, or “clock” gauges. The clock gauges are very fast acting (they are designed that way so you can see engine problems such as sticking valves). You will definitely need a damper if you choose a clock gauge. I would recommend using two side by side gauges for checking cylinder synch. The reason is, the cylinders are not perfectly independent. As one cylinder drops strength, the idle drops, and this will change the vacuum in the second cylinder’s manifold. It takes a bit of fooling around to get a cause-and-effect feel when you are turning the screws. Swapping gauges would make this difficult

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