All of the symptoms you describe sound exactly like the problem I fought
for several years in an '85 Westy w/1.9L .  In the end the problem was a
clogged radiator.  BUT your symptoms seem to include air in the system
also, sounds like the system has not been properly bled of air. Another
BUT:  You say you have a 2.1L wbx in an 85?  If this is correct then
part of the problem could be in the way the conversion was done.  I
can't list the details, but the plumbing for the cooling system on '86
and later 2.1L engines IS different from the earlier (83.5-85) 1.9L wbx
engines.  With the missing cooling components that you mentioned you can
suspect that the changeover to the 2.1L didn't take into consideration
the cooling system changes.