roasting coil wire
On MSD ignitions, the primary coil voltage is very high and is not compatible with most tachometers. The tach should be connected to a tach output wire, not the primary coil wire, and if it's not, it might be the cause of your issues.
http://www.msdignition.com/Products/Ignitions/Daily_Driver/6201_-_Digital_6A_Ignition_Control.aspx
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