One solution might be to obtain the ink and pen from a
strip recorder and
fit it to the plotter. These don't dry out as quickly and can be easily
unclogged with a small cleaning wire.
These plotters have a plastic cylinder on the carriage with 4 pens fitted
round it (it's a bit like the chambers of a revolver gun). When the
carriage moves to the extreme left, a spring bit of metal on the frame
hits a ratchet tooth on the cyclinder. By moving the carriage to/from the
left side 3 times (I think), the cylinder is turned by quarter of a turn,
thus sleecting the next pen.
Theres a magent embedded in the cylinder and a reed switch on the
chassis, used to detect the home position of the cylinder (black pen in
use, if you've fitted the pens conventionally).
The points of the pens fit into a spring metal circle with 4 holes that
keeps them off the paper. Across the printer, just behind the carriage,
is a rail operated by a solenoid at one side of the chassis. When
operated, it pushes the selected pen forwards, into contact with the paper.
Because of this consruction, it's almost impossible to kludge in some
non-stnadard pen.
-tony