well first measure the battery voltage
then measure the voltage at the solenoid
u should not drop more than 1 volt
even 1 volt is bit too much
if u ae gettin about 1 or more volts drop between battery and solenoid then fit a relay
straight from the battery then use the original starter solenoid wire to trigger the relay
the relay will then switch battery power to the starter solenooid
if u r droppin alot of volts u have a bad ignition switch or a bad connection somewhere