as far as the impco model J, all i do to test them these days is to remove the small pipe plug in the test port, and replace the plug with a barb fitting. i then install a vacuum/fuel pressure gauge onto this barb fitting and observe the primary pressure. Pressure should hold fairly steady at 1&1/2 to 2PSI with the engine running, and hold that pressure without climbing or dropping with the truck shut off.
if the truck is equipped with a VFF30 fuelock, remove the vacuum line to the fuelock with the engine running. the truck will stall as soon as the pipe between the fuelock and the vaporiser is exhausted of fuel. this time will vary from a second if it's simply a pipe nipple connecting the two, to thirty seconds if it's a long hose. if the engine simply sputters, or does not stop at all, the fuelock is stuck partially or completely open.
if the vaporizer housing frosts up in operation, check antifreeze level and circulation through the vaporizer body.
it is far simpler to replace the units than to rebuild them, though i have had success doing so. it depends on the labour rate and parts costs in your area.

mixer related faults may vary depending on what mixer you have but the model J and VFF30 symptoms are fairly universal and easy to diagnose. i do not like model K or Cobra units as the pin seat frets at the housing and they do not take rebuilds as well as the J units.