If you have a 1.9 L motor I would suggest changing the timing to 6 degrees
before top dead center, remember the TDC mark is the shallow groove , not
the v notch. that is 5 degrees AFTER top dead center.
Check the gap on the plugs, be sure to use antiseize on the threads,
..........After doing all of the above, you can check the bad cylinder by
pulling the plug wires off one at a time and then replace....all while the
van is running. The wire that has no noticable difference is the cylinder in
question.  If the engine runs rough after pulling a wire from the cap, then
that cylinder is obviously important and is pulling it's weight.  When you
pull a wire and there is no difference, then the motor does not know when
the plug is on or off,  that is your weakest cylinder.  I would also
reccomend a compression check when you pull all of the plugs. That makes it
easier on the battery and starter when checking the compression.  The
compression should be around 130 pounds or so......all of them.

Good luck....email me if you have any questions......

Malibu Tony 85' Westy  6 degrees BTDC......42 degrees full
advance......vacuum lines connected...