I have a mercruiser 120 that only runs when the starter is engaged or I run a jumper wire from the battery to?

Question

It runs when I jump a wire from the positive battery to the positive coil. What is wrong with it.

Answer 1

The resistor that the other poster refers to is to reduce the voltage while the engine is running thus prolonging coil and point life.  When your key switch is in the start position full battery voltage is bypassing the resistor, going to the coil to aid in starting.  Then when the key turns back to the run position, the electricity is fed though the resister.  It is just as common for the key switch to go bad as it is for the resistor to fail.  It could even be a wire or terminal problem on that circuit.

Answer 2

It will have what is called a ballast resister for the coil. By that I mean a 6 volt coil on a 12 volt system to enable easier starting as your cranking voltage will be around 9 volts.

This will look like a house fuse attached somewhere or be a long fat grey wire taped into the harness and it has failed. Find a friendly auto sparky and give him a bottle of his choice and problem solved