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Ok, so missing inputs, likely the pulse per RPM is right.
Today I win!! Fixed the jumpy Tach on my Vapor!!!!! - Suzuki SV650 Forum: SV650, SV1000, Gladius Forums suggests a Trailtech RPM sensor wire with built in resistor to tame the input, then go direct to a coil. If you're comfy cutting off the existing spade and crimping a new one on, twist in the Vapor's input, and use heatshrink to bundle the Vapor's wire to the coil feed wire so you're not relying on the crimp alone holding it together.