David, you didn't mention what year van you have, but by your
description it sounds like an air cooled engine.

Your timing scale may have gone missing and/or melted.  If you budget
allows it, get the aluminum one.

http://busdepot.com/details.jsp?partnumber=021119249E
http://busdepot.com/details.jsp?partnumber=021119249EA

The timing scale is mounted by the left 2 bolts of that also hold the
cover over the fan pulley on an air cooled Vanagon.  Water cooled is
completely different (they use the case split as the "0" marker)

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Now once you have the timing scale, you need to turn the engine over
to TDC of #1 cylinder.  ("O" mark on scale)

Easiest is to pull the valve cover off the passenger side of van.  #1
cylinder is closest to the front of vehicle.  Start turning over
engine with a socket wrench by hand until you see both valves
completely closed, actually if you are new to this have a helper turn
the engine (clockwise when standing at the rear) until you see intake
valve open, then close.  After intake closes and both valves are
closed is when you will have TDC.  The intake valve for #1 is the
second from the front.  You can determine this by looking at how the
exhaust header connects to the head, the exhaust is in the very front
and very rear of the head.  The two valves in the middle are intake
for #1 and #2 respectively.

It's important to understand that a 4 cycle engine makes 2
revolutions per cycle, this means that one time when the pulley
points to "0" you are at #1 TDC, the next revolution it will be at
#3, then to #1 again.  This is why you want to look at the valves,
you don't want to insert the dizzy when the pulley is pointing for #3
TDC

You can also search the archives for tips on adjusting valves, since
the procedure to find TDC of #1 for adjusting valves is the same as
finding TDC for inserting distributor.  (although your engine may
have hydraulic lifters so valve adjustment is not necessary).