Digital BW, The Print

I can’t get my yellow channel to print a good noozle check. I use a dedicated carbon b&w set from farbenwerk.com 
I tried everything known around the web (cleaning cycles...injection windex...cloth under printhead...). It produces 
Not a total clear noozle check but a very very light one (barely visible). I called the support and I think sending it in
for repair is not an option. I have it for little over two years now. Is it possible to create a QTR curve without the use
of the yellow channel (I don‘t have a densitometer or something like that), or should I consider another printer? I
Never did a curve by myself.

Marcus