QTR-Quadtone RIP

Attempting to set up one 'classic' - and proven - combination of 
carbon printing I equipped two brand new (old stock) Epson 1160s with 
fresh MIS UTFS/UTFSN inksets (both are fixed tone each). Test 
printing a proper picture via QTR and the included standard curves 
(1160 quad/UT) resulted in flat prints that seemed to lack any black 
ink (with both setups). The UTFS-print also displayed horizontal 
patterns like those stemming from blocked nozzles, although all 
nozzles were definitely clear.

I went on printing the ink limit determination chart at 100% which 
showed perfectly clean and graded steps for all four inks (both 
setups). I repeated at 80%, which seemed to represent the 'ink limit' 
best [while lying somewhere between the ca. 70% ink limit for black 
and the 100% ink limit for the two lightest black - something that 
confused me, by the way, i did not know which overall ink limit to 
proceed with]. In this print the black ink (only) would no longer 
print normally but produce unevenly horizontally 'striped' boxes 
(like an almost fully blocked print head)! Even the headline of the 
test chart and the text letters looked like that. I repeated at 90% 
and got the same result! I repeated with the other printer and got 
the same result. After that i tried QTR versions 2.5 and 2.4 and 
things did not change.

Then i defined a curve using a 100% overall ink limit and an 
individual ink limit of 100% for black ink plus the 100% black boost 
in later attempts. Printing the 21step gray chart with it resulted in 
the same problem as before, the darker steps all becomming unevenly 
striped where the black ink took over. Those steps that are printed 
with black ink only showing stripes only, the lighter ones showing 
line patterns to the extent that black ink is used (explaining the 
effect in the first picture print).

You will all understand my surprise and frustration, so I hope 
someone can give me an explanation/solution!

Thanks in advance
Goetz