I have a Datcon 100437 fuel sending unit with Ohm readings of 10.5 to 75.1 when testing with an Ohm meter.  this should work with my 56 Mercury.  I have the gas tank out and have hooked the sending unit to the wire that runs to the gauge and grounded the unit.  If I put the float adjusting arm at the empty position the gauge reads just past the 3/4 full mark.  If I put the adjusting arm at the full mark the gauge peg the needle.  If I put the adjusting arm at the mid point it again reads just past the 3/4 full mark.  If anyone knows what could be the problem please let me know.  The gauge goes to full when it is grounded.  I read somewhere on this site of an explanation of the sending unit but was unable to find the article.  Any help would be appreciated.  Thanks Larry

Larry