There normally no over compression. It is just that normal compression makes a four stroke motor hard to pull start. A decompression mechanism momentarily prevents full compression when a starter rope is pulled.
Mercury Marine has approved a number of models for use with lithium batteries.
Although not approved by Yamaha, Suzuki, Honda or Evinrude, a number of owners of those brands are using lithium batteries with no ill effects.
CDI unit gets its input from the charge coil. It "charges" a capacitor within the CDI unit. The pulser coil tells the CDI unit when to discharge electrical power to the ignition coil.
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